Currently using Kubuntu 8.04 with the latest Linux core files. Tried both Ubuntu and Kubuntu some time ago (as well as several other distros) but found that Kubuntu suits the way I like to do things so have pretty well stuck to it now. Still keep an eye on various other distros to make sure Im not falling behind in any area but nothing else really screams out at me at this stage.
Like you say, (K)Ubuntu is a very mature package these days and is truly very easy for any long-time user of Windows to come to grips with quite quickly. I think Billy should be worried ;)