I recently took the latest version of Ubuntu (9.04) out for a test drive and was impressed enough to migrate the laptop to it.
My favorite feature so far is the "throttle Windows" feature (my personal name for the feature).
Have you ever wanted to take your OS and just grab it by the throat and shake it like Homer does to Bart? If you combine Compiz and VirtualBox you can get the same effect. Just setup a virtual machine running the offending OS (for me it is XP [would be Vista except for the install size]), grab the title bar on the VirtualBox window and shake the crap out of it! Great fun for the whole family.
Seriously though, Microsoft should start looking over its shoulder...
A serious look at the latest from Ubuntu is coming soon.