I'm using Vista. It has brought me some incompatibility problems, but otherwise it works fine so far.
I dislike the Aero theme, except for the task bar and the window borders. Everything is really anti-productive, from its overly large dialogs, to the giant menus that take up so much more space. In addition, it seems Microsoft has no sense of font consistency. Aero only lasted a few days with me before I switched to the Windows classic theme. I wish I could use Windows XP themes though, because I have one that I modified to my taste.