There were some problems regarding driver support thought... but in the latest Ubuntu 15.04 most of them, maybe even all of them, are gone.
I've been using it for almost 1.5 years and can't recommend it higher.
Thanks!
On average lasts ~7 hours, less if you're watching a video.
Yes, sleep/wake up is working as expected.
I myself was very impressed when I got the laptop, I didn't expect such beautiful and fine tuned laptop from Dell.
Upgraded to 16GB of RAM and an SSD. The only thing I wish was that the screen was higher resolution than 1366x768, but otherwise it's absolutely fantastic for Linux. The dock works so seamlessly it's amazing.