I tried Vista Ultimate for 2 weeks. Then promptly went back to XP.
Same story here. That was on a machine that was bought specifically with Vista in mind (I'm going to have to support that OS at some point.) Some necessary stuff didn't run and I ended up having to beta a VPN client for Cisco just to get in. That, fortunately, works great but I don't think it's been released formally even yet.
The thing wasn't an unmitigated failure and there were some things I did like, but overall I'm thinking it's still a work in progress. IMHO, of course.