I took the plunge and upgraded mine tonight. I don't see any issues. I have about the same storage space after the upgrade as I did before. The phone seems much faster, even though it's not really a slouch to begin with. A couple of games took a few tries to startup, but eventually did. I haven't found any apps that don't work.
Two minor annoying things for me that people may or may not care about. The first one is turning sync mode on or off. I only turn sync on when I'm on wifi to save data usage. It used to be easy to turn sync on or off, now you'll need an app to make it easy.
The second one is the notification LED. When I'm home I always put my phone on silent and just look for the flashing LED for notifications. Lollipop doesn't seem to let you do that. If you want the LED to flash, it also vibrates the phone. I haven't found an app to fix that yet.
The battery life initially seemed shorter, maybe by 10 or 15%, but after a couple of weeks it seems to have settled back to normal. Perhaps it was just a calibration issue, or the phone was doing something in the background for a while after the upgrade.