I updated my Nexus 5 from Android 4.4.4 (ART) to Android 5 - Lollipop
I download the firmware from Google
https://developers.google.com/android/nexus/images
Now, Nexus 10, 9, 7 (Wifi only), 6, 5 and 4 are all having Lollipop
The testing is coming up soon, but little comments are (comparing with 4.4.4 ART mode)
1. Faster (or say smoother) and use less memory further
2. Build-in battery save mode (but slower). For normal battery life... still need to examine...
3. If you choose to auto-backup from Google, after flash it can restore all Android settings, users and installed apps automatically (can choose which apps you want to restore)

4. Better helper widget, and included flashlight
5. Have multi-user mode
6. ART mode is running in default
7. Better build-in keyboard
8. Seems Wifi reception is stronger