Windows 7 - I'm actually impressed!
Had a Vista machine running Vista Ultimate SP2 with 4GB RAM. Everything running fine but decided to put Windows 7 Ultimate on. Took about 30 minutes to install and hey presto - all the good things from XP and all the nice UI from Vista, only really fast and stable!
I have 3 Mac's in the house too and have upgraded them to Snow Leopard, I'm finding a couple of niggly things with Snow Leopard (64 Bit screensavers not displaying pics properly is one of them) but I'd still bang the following mantra.....
If you're at home - run Mac OS-X, if you're in the office run Windows XP or Windows 7, and if you're on the road then run Mac OS-X with Parallels or VM-WARE Fusion so you can get the best of both worlds!