I use Ubuntu as my primary OS and it has taken some huge leaps in being a usable desktop OS in the last few years. It's still not quite ready as a mainstream desktop OS, at least in my opinion, but its getting there.
Having said that, I would still suggest it over Vista to almost anyone. It's actually much easier to install and hardware support out of the box has been better that Vista on every machine I've ever installed it on. However there are a few things still holding it back such as the "non-free" plugins which could easily be confused as something you have to pay for. I've come across a number of things that are simple for someone with a bit of experience to work out but have left my mum and dad confused (such as obscure things like Java applets not printing unless you have the paper orientation set to something other than Automatic). This sort of thing is whats holding Ubuntu and Linux in general back for being a mainstream desktop OS.
For the moment I'm a Gnome user simply because I was never a great fan KDE 3.5 and KDE 4 is no where near ready yet. Despite the recent release of KDE 4.1 they still have a lot to get sorted. I'm waiting, hoping, that the KDE team are able to get things sort in 4.2 or even 4.3 because at the moment, although its getting there, it still doesn't feel finished. I feel at home in Gnome while KDE looks like a wonderful place to go on holiday but not somewhere I would be willing to live, yet