Tl;dr: The 2 Ups I had, died after 6 weeks and 5 months; the average lifetime of this device is 100 days, and customer support is horrible. I challenge you to Google "Jawbone up battery problem" before buying this product.
Had it for 6 weeks. The step counting was great and very motivating. The sleep tracking is nice in theory, but the sensor or algorithm that analyses it is not good - on nights when I had gotten up and walked 10 meters to the bathroom and back, the app would say I slept solid through the night and did not wake up once. How can it track walking around as "light sleep"? The best compliment I can give overall is for the app actually - it is gorgeous looking.
After 6 weeks the device suddenly would not charge anymore. A soft reset and a hard reset didn't help. Jawbone apparently only has a US site (there was no UK or european site), so I contacted their customer support. 2 days later, got a reply describing some steps I should take to reset the device, and they asked where I had bought the band and if it had been exposed to water (it had not). I replied that the steps had not helped and the band was still not working.
No reply from them to that. After 3 days, I made a new case since they hadn't replied to my first one. No reply to that either. Then I found out, via hear-say on Google+ (since Jawbone keep this e-mail adress secret) they have a EU Support email adress and I emailed that, this time mentioning upfront I bought it via Amazon.co.uk and describing the steps I had already taken. Within 1 hour, I got a reply:
" Based on the information you've provided, your Jawbone device needs to be replaced. Please contact the original retailer where you purchased the Jawbone device to request an exchange. Jawbone retailers are responsible for processing exchanges. "
That's it then - Amazon.co.uk gets the flack for Jawbones defective devices and pays all the shipping and other costs of replacing it. At least Amazon.co.uk turned out to have excellent customer service - after a quick chat they immediately shipped me a new Up and I could send the broken one back.
Avoid Jawbone. The MAJORITY of Up users have had their band die on them. The results of a poll among users (you can look it up on the Jawbone Up Google+ Community) was that their bands had an average life of 97 days (!) and users had been through an average 2.6 replacement bands. That sums it up.
--- UPDATE 15 FEB 2014 ---
Replacement band arrived and in use now, I will keep you posted on how long this one lives.... NOTE: I WILL **NOT** USE THE SLEEP MODE ON THIS DEVICE, since apparently the reason so many Ups only live 100 days, is the button you need to press for switching between steps mode and sleep mode - it's the button that's defective and breaks so soon. So I will use this Up just as a pedometer and without the sleep function. We'll see if that can make it survive longer than 100 days.
--- UPDATE 18 FEB 2014 ---
So NOW, after 3 weeks, Jawbone (US) has finally replied to my mails. As opposed to the "within 24 hrs" they advertise on their website... Well since they were not only incredibly late, but also less usefull than a random stranger on the internet was to me (THREE TIMES, in all my 3 e-mails, they ignored my question about getting an EU support adress) I won't be using and lost all my trust in them.
--- UPDATE 22 JULY 2014 ---
2nd band died this week after working for 5 months (had it from 15 Feb - 18 Jul). And I didn't even use the sleep function / touch the button at all on this (the replacement) band! This time it was a battery problem; suddenly the device would go from 100% to 0% charged in 4-6 HOURS instead of the 10 days it is supposed to last. I just threw it away, am not even going to get a new one from Amazon.co.uk (Jawbone themselves to nothing for the customer) this time, since the devices last only 2-4 months (AVERAGE LIFETIME OF THE UP REPORTED ON THE GOOGLE GROUP IS NOW 92.3 DAYS).