Would you buy a car if it had no fuel gauge? A phone without a battery meter? As humans, we like to quantify the world around us, but until now we’ve lacked the tools to answer the simplest questions about how we’re powered: Are you eating enough? Are you eating too much? What’s in the food you ingest?

The Healbe GoBe Body Manager is the first device that can track what you put in your body without requiring you to ever glance at a nutrition label.

The GoBe told me precisely how many calories I’ve shoveled in, and how many I’ve burned. 

Using three sensors and a lot of complex math, the GoBe I’ve worn on my wrist for a week has told me precisely how many calories I’ve shoveled in, and how many I’ve burned. And I’ve checked it against box labels and other fitness gadgets. I had been prepared to jump onboard with the vocal critics calling the GoBe snake oil. I won’t: The last week has been a total life changer. Not only does it work, learning how many calories I eat and how many I burn each day has me thinking about food and exercise in a completely new way.

Sadly, the GoBe is also one of the buggiest, most frustrating devices I’ve ever used. Here’s an in-depth look at one of the most exciting, yet still deeply flawed, gadgets on the brink of hitting shelves.

Next page: How the GoBe Body works

The GoBe wristband uses a combination of three sensors and a lot of number crunching to determine how many calories you eat and burn on a daily basis. The most unique numbers come from an impedance sensor, which sends high- and low-frequency electrical signals into your wrist tissue to measure changes in the amount of water inside your cells. It also contains a piezoelectric pressure sensor to measure your heart rate, and an accelerometer like any other fitness band to track steps.

From water-level data, the GoBe can estimate how much glucose is in each cell. By watching the levels rise and fall over time, it can determine how many calories you eat without the need to manually enter them in an app, and how many of those calories come from carbohydrates, fat, and protein. It combines this data with heart-rate information and some other info (height, weight, age, etc.) to spit out some key metrics:

Next page: Test results

Using Healbe’s GoBe Body Manager band has been a complete nightmare for a myriad of usability reasons I’ll explain in the next section, but as a miracle device that actually counts how many calories you ingest, it works.

For one week, I wore the band every day and simultaneously logged every food I’ve eaten in an old-school calorie-counting app called Lose It. The results were startling: The GoBe was within 10 percent of my calorie tracking estimates every single day of the seven straight days tested, and the average deviation was under 6 percent.

GoBe deviated further from this method when it came to the exact grams of protein, fat, and carbohydrates I ate each day, to the tune of 20 to 25 percent on average. For carbs and protein, GoBe typically thought I ate more than Lose It, but for fat, GoBe thought I ate less.

In one, I drank 120 calories of pure fat (1 tablespoon of olive oil), and in the other I drank 300 calories of near-pure protein drinks. In both instances, the GoBe proceeded to show the calorie count and makeup (fat, protein) with a fair amount of accuracy. Amazing, but true. For what it’s worth, Healbe claims that it’s most accurate when you eat balanced meals.

There is one big catch to using the GoBe: For it to work, you need to either tell it when you’re eating more than 100 calories of food by holding the button down for three seconds, or remember roughly when you ate.

It only takes seconds to tell the GoBe you ate a meal, but manually calculating calories by hand takes 5 to 15 minutes.

Over the course of the week, I’ve gotten better at telling the band when I’m eating, but I still often forget. When you do, it will crunch some numbers the next time it connects to your phone and asks with startling accuracy whether you ate food between, say, 12:20 and 12:40 p.m. today. If you say yes, then it logs the calories. If you deny eating, it will ask you if you ate at a few more times, then give up and delete those calories.

According to Healbe Managing Director George Mikaberydze, manually flagging meal times is necessarily because when you don’t eat for long periods, your body will naturally go into an emergency mode and release glucose into your blood, fooling the GoBe into thinking you just ate something.

I accidentally said yes to a few false readings and saw the GoBe add 300 to 400 false calories each time. Of course, this could also be energy that came from dense, slow-digesting foods you ate many hours before. Currently, there’s no way to delete calories you accidentally said “yes” to eating, but we’ve suggested this as a fix to Healbe.

In my experience, it only takes seconds to tell the GoBe you ate a meal, but manually calculating calories by hand takes 5 to 15 minutes after every meal, or a good 20 to 30 at the end of the day. If you didn’t eat packaged food all day and, heaven forbid, ate something that isn’t in a calorie database, you have to try to enter every ingredient in that burger, sandwich, or wrap and enter them individually. It’s difficult and almost impossible to keep up with on a long-term basis.

Counting calories automatically is amazing, but that’s only half the equation: Healbe tracks activity as well. It estimates how many calories your body burns, using basal metabolic rate information it gathers from your age, height, and weight; its own sensor data; and how much activity the band’s accelerometer takes in.

Next page: A compendium of complaints

I am in awe of what this technology can do. Impedance-based calorie tracking will change the game for the world of fitness and nutrition tech, and could be the “killer app” we’ve all been waiting for in a wearable.

Unfortunately, the experience of using the GoBe at present is beyond awful. No regular person would deal with the headaches I encountered with my review unit.

For one, it’s insanely difficult to charge. You have to set it upside down or sideways to coax a dim yellow light out of the finicky charger indicating it’s working, and even when you do, it won’t last. One of our writers with a second test unit managed to get his charging only by placing a five-pound dumbbell on it. I got mine to work through sheer will and patience, but it’s nothing a real owner would ever put up with. The sealed lid on one of our GoBe units even came off and it lightly shocked a DT writer.

No regular person would deal with the headaches I encountered with my review unit.

This is not a petite fitness band. It’s made of metal and it’s even bigger than the smartwatches we normally complain about. While I did get used to the bulk, it still annoys me from time to time.

Half the time we turned on the GoBe to a confusing “Init” message, half the time a  “Hello” message. Healbe reps tell us that “Init” is short for “Initializing” and only shows up when you start the GoBe from a dead charge, but it confused us frequently. The screen, which looks like what engineers in the 1970s would have imagined a fitness band might look like, doesn’t help matters. Reading the wide spread of thin LED dots tattooed on its façade takes getting used to. It also only displays horizontally, so reading the band like a watch is impossible. All text scrolls by, and the clock is permanently set in military time.

Three quarters of our review units had wrist straps without working buttons, which seems to be caused by the natural twisting of the band around the core unit. Even when it does work, the button is difficult to press and hold.

Only one of the three GoBe units we still have will sync with a phone at all, and it takes up to three minutes for it to send data to the phone via Bluetooth. If you haven’t been diligent about telling it when you eat, that slows down the syncing.

Finally, battery life is so poor that I’ve barely been able to test sleep tracking, because the battery is always dead by the end of the day.

GoBe’s hardware is a lot less refined than GoBe’s app, which is fairly pretty, but it presented significant hurdles as well.

For one, you must know a lot about yourself to sign up. To create a profile, GoBe needs your gender, weight, age, height, arm length, step length, “overall health,” blood pressure, and resting heart rate. Don’t fudge those last two, either: A wrong guess could have huge ramifications on the GoBe’s future readings. And despite tracking your heart rate every minute, it will never update your average if it changes.

While the app has a button to take your blood pressure, it just estimates it based on the blood pressure info you gave it, mixed with its own heart-rate data. I guessed my starting blood pressure wrong, and I definitely thought I was dying for a couple days when I consistently read my 150 / 60 blood pressure scores. Healbe needs to either remove this feature entirely, or warn people that it’s just an educated guess, before owners start showing up in emergency rooms.

The list goes on. Even if you’re doing everything right, every time you try to sync, the app tells you that you’re not wearing the GoBe correctly. There is a GoBe online portal at Healbe.com, but you can’t yet look at your data in aggregate, and the log-in system is screwy. On the rare occasion I’ve had the battery life to try sleep tracking, the app’s interface is so confusing that I don’t know what the data I’m looking at means. You can see your daily progress in the app, but there’s no way to view aggregate data for a week or month, or interact with the very small calorie graph shown.

Final page: Why this is going to be huge

I’ve never reviewed a product that has simultaneously caused me this much elation and frustration. When it works, the GoBe Body Manager has let me peek into my body’s inner workings and understand nutrition in a way nothing ever has before. What Healbe has done is truly astonishing.

I’ve eaten food every day for 30 years, but this past week, I finally began to understand how it affects me. Until now, no one — not an app, not my doctor, not my doctor’s dietitian — nobody could tell me how many calories are in the food I’m eating each day.

Healbe has not only accomplished that, it plans to take it even further. Future versions of the GoBe app will have dietary and exercise goals and guidelines, so you can better apply the newfound insight into human energy input and output.

I’ve eaten food every day for 30 years, but this past week, I finally began to understand how it affects me.

But getting it to work has been a nightmare. If Healbe ships this product in anywhere near the condition I’ve tested it, it will fail.

Still, there is reason for some hope. The Healbe already improved by leaps and bounds since we saw it in May 2014, and again in November. And the company assures us it is working to fix the charging and website issues (among other things) at a rapid pace.

Related: May 2014: The Healbe GoBe calorie-counting wristband is real, and we saw it in action

I’d love to see Healbe license its patented algorithms and tech to other companies, so you could buy a fitness tracker from Fitbit, Runtastic, or even Microsoft with built-in calorie tracking, but I can’t recommend the GoBe to ordinary people based on what I’ve tried. I’m not convinced Healbe has enough energy to reach the finish line, either.

In one form or another, this technology is going to change the game for health and fitness. Until then, I’ll patiently wait for this GoBe to sync.