I tried KDE with some other distros (I think I've tried nearly all of them, Gentoo was a total humiliation for me) but I really didn't like it, although some rave about it. It does have some great applications - Amarok for example, the music player which I'm running now and K3B the burner which is great.
They have some great video editors and image apps on KDE. But I prefer Gnome. And I haven't found a KDE app that I couldn't run on Ubuntu anyway.
It's probably just preference.
I did read somewhere that Konqueror, the KDE browser, is a bit iffy, I found it all a bit much and I'm using Firefox 3.0b5 (Hardy installs 3.0beta) and it's good but I have had Java problems with it and also streaming radio, 2.0 will do it, 3.0b tells me to get stuffed, it's not playing. I can put up with that though.
I'm quite happy with Gnome so Ubuntu rather than Kubuntu works for me. Oh, there are some spin-offs from Ubuntu of course - Mint is very good if you like KDE and it doesn't come with Java problems. If you're a multi-media maniac then have a look at Ubuntu Studio. I love the flexibility of Ubuntu and the whole open source thing. I'm only a user, not a hacker, but there are some clued up folks out there that are only too willing to help when I fiddle with something and break it. I have an AMD64 with XP on it which is my backup I run Ubuntu on a Pentium IV with plenty of RAM and an external hard disk for all my critical data so if I fiddle with the system and break it I don't lose anything important.
One thing about installation. I still haven't mastered partitioning so I just dump the new distro on the hard drive which means bookmarks and emails all gone of course. I think Hardy may be better at guiding partitioning, though I haven't tried. I did have Ubuntu (Dapper) living with XP but in the end I thought stuff this for a game of soldiers and just went Ubuntu totally on this machine.