I was so impressed with KDE4 in Kubuntu Hardy that I've reinstalled it to my Kubuntu laptop. The issue I had with Pidgin appears to have been fixed (although many Gnome apps still don't look that great in it).
I've added the extragear-plasma package and put in a few widgets, as well as changed the wallpaper, and here is the result:
It really is very impressive - can't wait to see how KDE4 develops over the next year or so! I imagine we can expect plenty more widgets - in fact, from KDE 4.1 it should support Mac OS X Dashboard widgets, which is good as I would like to use the BBC Radio Last.fm widget so I can listen to 6music and scrobble the tracks to my profile. I may even see if I can convert it into a KDE4 version myself as I have downloaded it and will see how simple it is. I suspect I won't be able to, but if necessary I can wait for KDE 4.1.