I have upgraded from Android 4.4 KitKat to Android Lollipop on my Nexus 5 mobile phone over the weekend. Some observations so far:

• Overall interface looks slicker and prettier
• The system seems slightly faster
• Automatic battery saving regime sounds nice
• I could say both good and bad things about the new notifications system:
  • It tries to be more intelligent
  • It is less useful when the screen is locked - I find tray area icons a much more informative and laconic option
  • It is less secure from privacy point of view - the email notifications clearly show your email address even with screen lock - I think it's a privacy oversight
  • Multiple notifications can clatter your lock screen
  • All these new features are almost not configurable
  • Multiple notifications can clatter your tray area
• A number of applications failed to work correctly after the upgrade. Reinstalling them fixed most of them. My SSH/SFTP server is still broken, so I sent an email to the developer. We'll see if it's fixed in some reasonable future
• I am still able to connect to the phone via MTP and transfer files
• I find the extra swipe one needs to perform before unlocking the phone quite annoying and counter-productive
• Some people have complained they couldn't reach me by phone. It's not clear if it's my device' fault or they had issues on their end. A number of other people could call me just fine. I was able to place calls without issues
• It sounds like the OS clips audio track at the end during playback. I haven't seen a setting which would control it