I'm going to tell a long story in a second, but here's the TL;DR version: The $180 Up3, Jawbone's latest wrist-worn fitness tracker, is very few of the things it promises to be. It's wildly ambitious, and spectacularly flawed. Even after Jawbone took six extra months to improve and perfect it—thanks to production delays, because waterproofing is hard—it didn't get it right. You shouldn't buy it. You should buy the $100 Up2 instead, which tracks your steps and sleep just as well. Or buy a Fitbit, or any of a thousand other basic fitness trackers. Everything else about the Up3 is snake oil.

OK, moving on: Let's talk about your heart. The ol' ticker. This may shock you, but it's important. And it's a new focus for a lot of fitness trackers, from the Fitbit Charge HR to the Microsoft Band to even the Apple Watch. Your resting heart rate is a particularly useful number—it's the point where your heart pumps the least blood, usually right after you wake up. It's a really good indicator of your overall fitness level, particularly as it changes over time. Among other things, there's even some evidence that if your resting heart rate suddenly spikes, you're about to get sick.

The inside of the Up3's band has five square metal contacts, which use tiny currents against your skin to get heart-rate readings. But for all that tech, the band doesn't care for anything but your resting heart rate, and I don't know why it measures even that. The first night I wore an Up3, my RHR was 55 beats per minute. That seemed good! A few nights later, it was 74 bpm. Clearly something was wrong that night, but what? Answering that question is supposed to be Jawbone's big plan, the thing it says it can do now that you have all these sensors strapped to your wrist. It can measure everything from steps and sleep to ambient temperature and respiration rate.

Not that it does much good. The Up3 hardly shows me any of that data, and it doesn't do much with it. Half the time, it's not even any good: my Up3 seems to think that my 20-minute train ride every morning constitutes vigorous exercise. This thing is connected to my phone; why can't it check my GPS, or my calendar, or figure out what I'm actually doing? All this data is supposed to make the Up3 predictive and proactive, telling me things I need to know before I'm even aware of them. I already know I'm supposed to eat better, sleep more, and hit my step goal, and I even like having a device that reminds me. Yet you use the Up3 for a week or so, and you realize: this thing's still just a step and sleep tracker. A good one, sure, but I'm not cheering for a good step tracker. And you don't need to spend $180 to get it.

Josh Valcarcel/WIRED

The Up3's heart rate readings are relatively accurate, or at least consistent with competitive devices, but that doesn't matter until Jawbone figures out what to do with the information. For metrics like steps and sleep, there's at least some benefit in simply tracking them. It took all of one day before I started going way out of my way to hit my step goal, and feel the Up3's joyful buzzing on my wrist. As soon as I figured out that I don't get enough deep sleep at night—the one actually useful result of the Up3's new sensors—all I had to do was Google to find out how to fix that. (Turns out stretching is a good thing.) And I am hopelessly in love with the Up3's Smart Alarm feature, which vibrates to wake you at the perfect moment in your sleep cycle. When I use it, I wake like a Disney princess, smiling and stretching joyfully into the sunshine while birds flutter sing-songily around my room and select my outfit for the day.

Jawbone's app is perfect for those simple things. It doesn't overload you with data, but makes it easy to see how you're doing against your goals. Jawbone constantly has ideas for how to sleep better, too, or a challenge to get in bed a little earlier tonight. It's like a super-earnest wellness blog in push-notification form. The app is available for most platforms, and hooks into a huge number of devices; Jawbone's as much a software company as a manufacturer, and it's a great hub for your fitness data.

Unless, of course, you're the type who wants more and better data—you know, the kind of person who would drop this kind of coin on a fitness tracker. Then, the app becomes infuriating: it takes four taps and two confirmations to log a glass of water, and you're not going to learn anything about your sleep beyond not-that-accurate readings of deep, light, and REM sleep. OK, Jawbone, I woke up twice last night: tell me what I'm supposed to do about it. Oh, and those 43 minutes you say I was awake? I was showering, because it's the morning, and you should know that because you are my alarm clock.

Every single thing Jawbone does well is also inside the Up2. For all its bells and whistles, the Up3 does nothing else. Other than "exercise more," I don't know what to do about my resting heart rate—and I don't need to spend another $180 to know that. My unused gym membership is reminder enough.

In fact, the only real improvement over the much older Up24 is the design. Jawbone's old devices, like basically every other fitness tracker, were big, rigid, clunky bangles that scream, "Technology!" The Up3 and Up2 aren't exactly high-end jewelry, but they're about the most wearable wearables I've ever seen. Jawbone worked hard on making it smaller: it wants you to wear the Up next to your smartwatch, not instead of it. And that looks crazy, but only a little bit. For one thing, they're finally small enough that I don't have to take my Up off when I sit down at my computer. They're incredibly simple—and identical, except the Up3 is a little larger—with a rigid part that goes on top of your wrist, and a super-flexible rubber band. It's super durable and water-resistant, but not waterproof—a concession made in production, and the cause of the product's delays. It comes in black and silver, but trust me on this: get the black.

Josh Valcarcel/WIRED

It's a "one size fits most" device, with an adjustable hook-and-eye sort of clasp. It's awkward to put on, especially because you're supposed to make it tight enough on your wrist that the sensors press against your skin. Once you get it on, though, it holds tightly—or at least the second model Jawbone lent me did. The first one came off a dozen times in a day, including while I slept. I missed my alarm by 90 minutes after I got the Up3, because there was nothing on my wrist to buzz me awake. Normally I wouldn't mention a hardware defect like this, because there's always a few that don't work right. But Jawbone has such an ugly history with imperfect hardware, and the issues with the Up3's buckle were so frustratingly expected, that I can't just write it off.

That's what's so frustrating about Jawbone: It has all the right ideas, and just never quite nails the execution. I love the idea of a fitness tracker that collects and collates more data, that can tell me more specific and actionable things over time. On paper, the Up3 is exactly the thing I want, more so than any of its competitors. But even ignoring the fact that it doesn't turn more data into better data, the experience of using it is a mess. I never figured out the double-tap-then-long-press method of switching between Active and Sleep modes, because most of the time it doesn't work. I also can't figure out why it doesn't switch automatically. I can't stand that some days, for some unknown reason, it just doesn't track my sleep. I like that the battery lasts a week, but not that it doesn't really warn you before it dies, or that it has to sit just-so on the awkward induction USB charger or else it won't charge at all. This is easy stuff, or at least it should be.

I want Jawbone to succeed, because I want the device the Up3 was supposed to be. Instead of a revolutionary fitness tracker, though, I've been wearing something that feels like a Kickstarter prototype that they swear will be better before it ships. So I waited six extra months for it to ship, and it's still not as cool as advertised.

Maybe the Up3 isn't possible yet. Maybe you can't build something that powerful, in that small a package, for $180. Maybe no one can. But I know this: six months ago, Jawbone promised the world. Then it built a step tracker.