Interesting, I found this to be the second best update ever, behind the jump to Kitkat. The phone is so fast now that it is like I just purchased a brand new model. All my previous apps still work except for one non-critical one that I planned to remove anyway. Many very welcome tweaks have been added. Too early to tell about battery life, but my first impressions are I see no change. As far as Gallery is concerned, I just use the Photos app, but you might need some of the features that were in the Gallery app?
Then again I waited until yesterday to install Lollipop. I installed the 5.0 factory image (with the modified flash-all.bat so it retained my apps/data), didn't even try it out so I can't report about any bugs, then immediately installed the 5.0.1 factory image the same way; I was waiting for everything to stabilize as I always do when a new release is issued, meaning waiting for the next update with various fixes to be issued that always comes out (aka 5.0.1 in this case), an updated TWRP, a fully working SuperSU, and time for apps to get updated.