It's not often that I go out of my way to praise something from Microsoft, but I have been using Windows 7 Professional for a week or so now, and I must say this is easily the best desktop operating system I have ever used.
I bought the upgrade edition, but used the Custom Installation option, wiping everything (in my case, Vista and Ubuntu) from my hard drive. The installation was spectacularly uneventful - it just worked. I didn't time it, but it can't have been more than 30 minutes or so from start to finish: and no issues.
I haven't explored all the features yet (e.g. Libraries), but I'm truly impressed with just the basics - the whole taskbar implementation, the new tray/notification area, the clean desktop - and the fact that it feels much faster than Vista, though I've absolutely no hard evidence to back that yet.