I pulled the factory image and loaded Lollipop from scratch with a factory reset. So far, it's been very smooth sailing. Occasional lag, but about the same as I was running into with 4.4.4 anyway with occasional LONG lags when loading large apps into memory (encryption).
I'm not a huge fan of 5.0 overall - mostly because of all of the changes to the setup screens (Google should never take design cues from TouchWiz - just sayin') and how bright everything is - I don't favor so much brightness.
It's not something I would encourage anyone to seek out just yet. It appears to work OK as long as you accept that a factory reset is a Really Good Idea. This is an update heavy in cosmetics.
The two major improvements are:
1. ART - new runtime that replaces Dalvik to run apps. ART pre-compiles apps instead of just in time compilation. Installation takes longer and apps take up a little bit more space, but since you don't have to compile the app every time it runs many apps run faster. However, ART is new - it has previously been available only by activating developer options on select devices and enabling it manually. As a result, some apps still have poor support for ART.
2. Device encryption on by default - makes your data more secure, but the encryption layer really slows down read/write to storage, which can cause lags when loading large apps or anything else that reads or writes large amounts of data from/to storage.