Friday I upgraded my system from the previous version of Ubuntu to 11.4. So far upgrades have generally gone well, this time not so much. The new interface is awful--appears to be optimized for Netbooks and other tiny screen computers--I've got dual 23" monitors, and the controls are all the way to the left, combining both the controls for launching apps, and the controls for accessing the already running apps in one bar. This isn't a dealbreaker, since the old interface is available by choosing 'classic mode' when logging in.
More important is that on the same system that was perfectly stable I've had several crashes and other glitches in just a few hours. Had to uninstall a plugin to Rhythmbox, othewise launching it would crash the system. Trying to access a CD logged me out. In my first attempt to write this post the backspace key got stuck, and unplugging the keyboard did not help.
I'm backing up my home folder now, and I'm likely to do a reinstall of the previous version. I'm getting more irritated with Ubuntu with each release--Instead of being the distribution that just works, they keep making fairly drastic interface changes with little apparent justification.
If you see distribution update in update manager, it's probably best to skip it for now.