A few days after the official launch, bugs on the latest Android OS, Lollipop, starting to show up on Nexus 5, one of the device that received Lollipop, immediately. One of the biggest problem, battery drain, has been reported on Android Developer forum.
Google hasn't mention whether this kind of issue also occurs on their tablets or other version of Nexus. But the good news is, looked like that the latest Nexus 6 and Nexus 9 didn't experience the same thing.
I'll stick with KitKat then.