I'm doing it because I hope to end up with a legal retail version by using the beta until the launch. I certainly wouldn't pay for any MS product, but this helps me avoid paying for another Windows license in the long run. And if it doesn't happen that way, I haven't lost anything.
As it is, the beta is apparently better than Vista was, probably because theere aren't any major new features.
The main features are the taskbar (Lol Mac clone), HomeGroup (totally incompatible with other Windows versions and OSs so worthless), some new window minimising shortcuts (that's a feature?), IE8 (Who cares, FF is better and IE8 combines the bad of IE7 with lots of bugs, such as major websites failing to work), and DX11 (Which you need hardware from late 2009 for).
