is kubuntu a seperate download or just a different gui for it?
The only difference is that it uses the KDE GUI manager instead of Gnome. The underlying kernal is the same, but there are a few differences in which software works best on each environment. If you're running one *buntu it's easy enough to put one of the others on and swap between them. The main reason I use KDE is that most of the time I'm surfing the Internet and listening to music, and Amarok for KDE is hands down the best music player software I've ever used.
I keep thinking of using ubuntu/kubuntu, is it that good?
Personally I love it - it's definitely the most user-friendly and non-techie GNU/Linux variant I've used. Over the years I've used a ton of different operating systems - Amiga, every flavour of Windows, Mac OS7.6 up to OSX, Solaris, Red Hat and now Kubuntu. I don't think I'd use anything else now. Just need an Xbox 360 in my room for the occasional blast on EA Skate or Halo 3 and I'd never need to leave the house! :D