On my laptop I have 3 hard drives and 4 "distros" (well, one is Windows XP); Ubuntu 10.04 (primary), Ubuntu 10.10 (testing) and Fedora 13 (becoming primary), each with their own state of working since this laptop has an Intel Corporation 82852/855GM Integrated Graphics Device (rev 02) for video which has a kernel bug that bit when 10.04 came out.
I was able to apply a patch in 10.04 to make it mostly work, while 10.10 worked better out of the box yet it lacks in other areas.
Out of all of them Fedora 13 has been working this best, giving me desktop effects, handling external monitors and playing video. Ubuntu has varying states whether it doesn't handle external monitors (10.04) or desktop effects (10.10).
At one point I had openSUSE installed, but some update trashed the video in my system to almost make it useless after a short period of time.
Before I removed it, I tried updating it to KDE 4.5 two times. Both attempts failed and so I was accepting the fact I would have to wait until openSUSE 11.4 (next year) or later to try KDE 4.5.
I laugh because I thought openSUSE was a KDE distro while Fedora was a Gnome. Seems they work opposite on my system.
With Fedora I have been running the stock KDE. I was pleasantly surprised to find that after my 53 updates were installed (it's been a while), I have ended up with KDE 4.5.2! And even better, it looks like everything still works!
So this has been probably one of the most painless updtaes. Now I am nervous about upgrading to Fedora 14, but I'm thinking "if it ain't broke, don't fix it", so maybe I'll wait until 15.