I've been using Linux as the main OS on my PCs for years. I own an older Acer laptop and a 1.6Ghz Atom netbook, and use Ubuntu 10.10 on both, but I also have Icaros (Aros) on each of them which I use as a secondary OS for research, testing and development. I think as a user OS, Ubuntu has everything you need really, except maybe commercial games. But anyone who has come over to my place and had to use the computer has had no trouble using Ubuntu, finding the program they need and using it to do what they want (usually opening a browser or IM client).
I'd love to try setting up OSX on my netbook if that's possible, I've seen it running on other Intel Atom/GMA combination Hackintoshes before.