I upgraded to Ubuntu's latest yesterday - Hardy Heron. So far so good, I upgraded only because I'm an upgrade junky. Gutsy was working very well and did everything I wanted it to.
The stuff that I thought would give me the worst problems, the Nvidia video card, is actually working fine. The stuff that I wasn't expecting to have problems with (my keyboard and window manager) are being sort of a wonky. The number pad isn't working, and in fact pressing the key I thought would make it give me numbers ended up locking the keyboard all together (I had to log out and back in). Also, the display stuff doesn't want to use Emerald - and I haven't looked up yet whether Metacity is now the default for Compiz, or if somehow my local settings are messed. I should add a new user and log into it to see what the default brings up.
My computer fan seems to be running all the time now. I'm not sure if this is because the OS is now requiring more resources, or if the new power management stuff is "helping", but since I like my machine psychotically quiet until it absolutely needs stuff like the fan, that's annoying me at the moment. (I've actually installed one of those super-quiet fans in this box to make sure it's as quiet as it can be).
The upgrade took a long time - I probably should have just backed up my home directory, grabbed and .iso and started from a clean install, but it ended up being easier this way, if a little longer - downloading the packages one by one, then doing the update in the background.
Firefox 3.0 is the new default - which means none of the plugins I use daily to do development (Firebug and Web Developer) are working. Also, I *hate* the new dropdown box in the address bar. Nice try guys, try again - it sucks. It looks like there's several 'prism' based apps in the repository for gCal and Twitter which might be interesting to look at, or I can get the plugin and make my own dedicated web-based desktop apps. That's pretty cool.
All in all though, it was a successful upgrade. Usually upgrades can cause boot problems, screen resolution problems, network problems, etc. but this time everything went really well. Ubuntu is rocking more than ever! If you're still on Windows or a Mac, now's the time to go free baby!