I'm a computer tech at my University so I have to keep up with all the common OS' just due to my job, and I've got my main Desktop with Vista Ultimate, I'm getting a new MBP with Leopard, I've got an old Dell Optiplex I got for free with XP Pro, and a Dell Latitude D600 I also got for free with Ubuntu 8.04 on it. I use then all pretty equally, depending on where I am and what I'm doing... It's really nice to be able to use them all and when you learn one trick to see how it crosses over to the others. You also grow to miss some features though, example: Virtual Desktops. Linux pioneered it, Ubuntu, Fedora, Suse, etc have all had them since they started, Leopard finally just got it, and for a few years XP got the Microsoft Virtual Desktop Manager Powertoy. But Vista still has no elegant way to use multiple desktops, and the 3rd party ones just stink. Anyways, what do you use? Presumably OS X is going to be the favorite of most up us here, I'd say its a tie between that and Ubuntu, I'd switch over to Ubuntu mainly but theres still some features its lacking, seamless Cisco VPN support has been an ongoing issue for me, as well as openoffice lacking some of the more advanced features and .docx, .xlsx, and .pptx support, but thats all to come I'm sure.