You can get a free copy of Windows 7 to play with — it’ll be good until March 1, 2010.
Windows 7 Release Candidate Customer Preview Program
I wouldn’t replace my operating system with a Release Candidate (that means more than beta, but not yet for sale), but I’m using it for Windows on my Macs. It seems better than either XP or Vista, but I don’t begin to put any stress on it.
It’s a 2.4 or 3.0 GB download depending on whether you need 32 or 64 bit.
Our old friend Ed Bott (he used to comment around here) writes what to expect from Windows 7.