Got one for my girlfriend and me two days ago. Overall I like it (and she does too). So here's my quick thoughts on it...
Comfort:
Most of the time I forget that its on (which is a good thing), but I did have problems sleeping with it. I sleep with my hand under my pillow and woke up with an aching wrist. The next day I made sure not to do the same. Not really a huge deal imo. I'm assuming future generations of the device will be smaller and wireless.
Move:
So I mainly bought this so that I can track how active I am from day to day. As an extra bonus, my girlfriend and I are constantly monitoring each others activities, policing our lazy habits.
The calorie count, steps, and distance traveled are not always perfect. It calculated 2.26mi on a 2.1mi run, with 260calories burned (probably closer to 200). Another note to add is that small movements seem to add to the steps, a 2 hour car ride for example gave me 86 steps.
Overall, it works for me. In my eyes the small errors in calculations aren't really a big deal when tracking overall daily activity. If everything is wrong to the same degree, then I still get pretty graphs that say yesterday I was more lazy then today. I am constantly doing small things to try to get more steps in to beat my girlfriend =p. Will the motivation last? I don't know, but for now its working.
Sleep:
Not much to say here, it tracked my two nights of sleep pretty accurately. It even noticed I woke up to get a drink of water. The vibration alarm is very pleasant compared to the loud music blaring from my iPhone. As i mentioned before I had one night that I slept on it and woke up with wrist aches, but this can be avoided.
Eat:
From all the reviews, this is the worst part about the Up. It really is, but there is some positive to the eat function. With supportive friends (or girlfriend) you can monitor what each person is eating. I found myself not eating at McDonalds the other day b/c i didn't want to post it for my girlfriend to see. Neither of us are overweight, but we like to stay fit.
Challenges:
I thought this was a bit of a gimmick (kinda like the Nike+ stuff), but since its so easy to accept challenges on the iPhone I actually took the challenge and rubbed it in my significant others face =). If you have friends, it definitely makes it interesting. Otherwise, doing challenges may get a bit boring.
The App:
Right now...really buggy. I can list a handful right off the top of my head. Sometimes it fails to sync (I unplug it and plug it back in multiple times). It forces my iPhones volume to max and I have to hard reset my phone. App doesn't display statistics from time to time...etc etc. Yes, its buggy...but it will get better.
Conclusion:
If I weren't a gadget nut, would I get one now? Probably not. As someone mentioned before this seems like a trial run and the app definitely has a beta-feel to it. Maybe the next firmware/software update will add in much of the functionality I want. Its got me motivated right now, so its doing its job. Whether its worth 100$ or not is up to you.