That PC is the 'family' computer - so basically all it's used for is Surfing, streaming music through iTunes and instant messaging.
It used to run ubuntu but everyone complained about how ugly it is - and I wasn't going back to XP. At least with Win7 I'm in the clear anti-virus wise, no need for that.. yet.
However, it's been a while since I've used it (I have 10.4 and XP machines for the most part), and I get the feeling that Gnome has benefited from Ubuntu far more than KDE has from Kubuntu, being sort of a bastard stepchild at best.
KDE's Konqueror has features that I still find myself wishing for on my Mac on a daily basis. (E.g., the visual view-by-size thing and built-in sshfs are my top two.)
I've not seen any compelling reason to upgrade my Macs from 10.4, so when they finally hit EOL I may seriously reevaluate Kubuntu as my main desktop, see if it's kept up reasonably well versus the mainline Gnome version.