Yesterday I upgraded to Ubuntu 9.04 (from 8.10), and I must say I'm quite impressed, it feels a lot more stable than previous versions, and a lot more mature.
In 8.10 Wi-Fi wasn't very stable, I kept having to boot to Windows and then boot back to Ubuntu for it work, and now it just works.
The new themes they've added are really good, I'm currently using "Dust Sand" which has a Mac-ish look it without being a Mac-clone.
The new notifications system is very nice:
They hide when you move the mouse over them (so they don't interfere with what's underneath them).
Also, they've finally added the ability to resize a window with Alt+RightClick instead of only Alt+MiddleClick (it's a real pain when you're using a laptop or when your mouse wheel isn't very good...), I remember writing a patch to metacity a couple of years ago and I'm glad they finally fixed this bug.
If you're using metacity just open gconf-editor and enable
If you're using compiz (if you enabled the visual effects) then:
sudo apt-get install compizconfig-settings-manager