Update: It has come to my notice that many people do not know about this (and other problems listed in comments of this post). Please digg and reddit this post so that more people can be made aware of it before they upgrade unknowingly.
You should not upgrade to Ubuntu 9.04 (aka Jaunty Jackalope), released a few weeks ago, if:
You have an older ATI graphics card (prior to R500, .e.g xpress 200m)
You like to have 3D support (read compiz/games, etc)
This is because of the following reasons:
Ubuntu Jaunty Jackalope has the new version of X.org server (1.6)
The open source graphics drivers for ATI do not have 3D support
The only version of the ATI properietary drivers (fglrx\Catalyst) compatible with X.org server 1.6 is 9.4
Support for older cards (mentioned above) was dropped from fglrx 9.4
So, basically right now you are in a soup and it seems to me that this is going to remain this way unless someone figures out how to backport x.org 1.5 onto Jauntu, which is going to be a hell of a task, IMHO :P
But, if you are a simple person, who looks beyond the gloss of the 3D and marvels at the beauty that the rest of the system holds, then go right ahead and dive in :)