I'll start off by saying, I already use Vista.  I use XP.  I use Windows 2003 Server and Windows 2008 Server.  I beta tested every version of Windows since Windows95, so I've had my experiences with beta operating systems.

Windows 7 is nice.  The taskbar takes a little getting used to, but it works.  The updated versions of the included apps aren't anything to really write about, they are still too under featured to evaluate in comparing operating systems, but they are nicer looking and work very well.  Mind Sweeper now requires a GPU for it's fancy effects, but they aren't required to play the game.  The HomeGroup feature looks kind of nice, but I'll have to setup another VM to see how it really panes out.  MediaCenter updates are nice too, but it crashed while I was playing with it.

 All in all, it is nice and stable, but I think I'll install the 64-bit version on one of my laptops to give it a fly one a more general usage, including some development work.  My experience in the last three hours hasn't differed from that of the 98, XP or Vista betas.  A couple of "oh cool" moments and very few problems setting it up, but it will need to be really used before the real evaluation occurs.  Windows98 beta worked great until you selected a video in explorer.  XP beta worked great until you tried to install certain apps.  Vista's beta was actually on of the more stable betas, but driver support and some legacy apps didn't work.  Windows 7 looks like it will be more like the XP beta since there are fewer architecture changes.