They are all a little different, but so far here is my take on them.
Jawbone UP (The first one that came out, not the UP24). I bought this one when it first came out and a month later they pulled it off the marked because they kept breaking. In the end they replaced it with the new version when it was released again. This was before the UP24 came out. The step counting seemed pretty good, I remember it being pretty accurate with regards to distance when I took walks. The idle alert was a nice feature to remind you to get up once in a while, especially at work. You could use the button to switch between sleep, nap, activity and regular mode. The nap mode was pretty cool, set it for 20min and it wakes you. The sleep shows deep and light sleep which was nice but I found it usually didn't pick up the times I woke up during the night. The smart alarm would wake you during your lightest sleep within a time window (mine was 20min). In the end I switched to the Fitbit because the LED burnt out and the button wasn't working great, having to plug it in all the time to sync wasn't great and the band gets snagged on any long sleeve you wear.
Fitbit Flex. Step wise it's about the same as the UP. The sleep only shows restless and awake but it would pick up the times where I woke up during the night when the UP didn't. Doesn't show deep and light sleep but I still preferred the Fitbit. Alarm was at a specific time you set. I found it much more comfortable than the UP, and it doesn't get caught on things. Changing modes requires you to tap it which takes some getting use to. It would also sometimes change to sleep mode if I was cooking and chopping food up but it didn't happen too often and it vibrates when it happens so I usually noticed it when it did.
I switched to the Fitbit over a year ago and just recently put the UP back on to compare it to the pebble. Here is today's stats so far:
The sleep is written as "Sleep/Deep Sleep"
Between the Fitbit and UP I do prefer the Fitbit Flex. I find it more comfortable and more durable than the UP. Keep in mind I haven't used the new UP24 so my experience is on the older non Bluetooth version. The app for the UP is much nicer though. I am quite impressed with the Misfit app on the pebble, the steps are a little lower than the bands but the sleep accuracy is pretty good considering it just does it on it's own. It's hard to analyse it since you can't sync to the app yet. It will be very interesting if the UP for pebble adds sleep tracking as well.
Hope this helps.