Ars Technica has a great review of KDE 4.0.
I installed the Kubuntu/KDE4 Live CD on my laptop (using Parallels), and overall I agree with the review; it's a great step forward, but not quite ready for everybody. I've ben a GNOME guy for a lot of years, but I think I'll be switching to KDE in the near future.
I'm having some trouble with screen repainting (it doesn't), but that's most likely a Parallels/Xorg problem. Otherwise, provided that you follow the directions for Ubutntu 7.10 as a Guest OS, it works fine.