Vista SP1 RC1 has just been released for public consumption.  If you want to try it out, you can do so here.  I'm running this on most of my machines without incident.  This includes my Media Center at home.  So, from my few data points, it seems quite stable.  Don't expect any major changes in functionality.  This isn't XP Service Pack 2.  It's more like a traditional service pack.  Lots of bug fixes, but it's not being used to roll out big new features.  If you want to read about it, check out Paul Thurrott's review.  Remember, this is the release candidate, not the final build.  If you install this, you'll have to uninstall and upgrade to the real one whenever the final is released.