I have been a "Jawbone UP* user for the past several years. Started with the UP, and had the UP2 and UP3. While I absolutely loved the size and the band for the jawbones -- unfortunately after about a year they would stop working for whatever reason (the last one wouldnt charge, I never wore it in the shower, but the charging section started corroding I guess just from the sweat over time during the workouts).
So - to compare vs the Jawbone --
Size & Screen - while its a little bigger, I never needed the screen, but I do enjoy having it. The screen does make it easier to monitor progress vs having to open up the app. And, even when the app doesn't sync, you can see the rounded step count on the screen. And, its nice because you can see the next "alarm" so when you are lying in bed and cant remember if its on or not, you can just tap the screen vs reaching for your phone which I keep downstairs.
Band - the band definitely takes some getting used to. It probably took me a good 10 minutes the first time to get it snapped in. I could not get the band snapped by using my thumb and finger... I finally found that i needed to essentially line up the silver piece to the side of my wrist bone and press extremely hard to get it to attach. And, you would think that by needing to press so hard it would be sure to stay on... that doesn't seem to be the case as its already fallen off a time or two even though I've checked when it was put on, and it was as far as it could go. I definitely prefer the clip on the jawbone vs the band on the fitbit. While i don't want a watch looking band, I unfortunately will likely need to switch to one to prevent the device from falling off unnoticed.
Step Count / Distance Syncing -- perhaps my biggest gripe. The jawbone was dead on accurate out of the box (all three i had). I walk and run on a treadmill and on run/walks of my usual 5-6 miles, it was never more than 1 TENTH (thats .1) miles off. Very accurate. For the Fit Bit -- totally different story. The ONLY time I have found the fitbit to be accurate for me is when I'm inbetween about 3.5-6 MPH. When I'm just walking around the house -- it tracks MORE steps than I take. But, the biggest gripe -- if i run 6 miles (usually ranging between 6-7MPH on the treadmill), it usually tracks about 5-5.25 miles. So, the faster i run, the less it counts.... kind of annoying when you know you ran 6 miles to see nearly a FULL MILE lost. I'm sure there is some way to sync it - but given its very variable in if its under, over or accurate -- I feel that if i synced it to the full run, then it would likely really be over for the rest of my daily activity. Very unfortunate as I never had any syncing issues with the jawbones
So - as a Jawbone and now a fitbit user... I think it covers the basics... it generally keeps track of the activity, i find myself liking the screen and its not TOO much bigger than the jawbone, and it has the other functionality i was looking for such as reminders to get up through the day (desk job) to at least walk around a little bit each hour.
Given I've only had it a few months -- will be interesting to see if it lasts until next year :)