I've now been using Vista for one week. I've endured a few bugs, but overall the experience has been good.
Most irritating bug: mailto links broken.
I'm not sure if some software I installed broke this, but it took me a while fix this. Whenever I clicked on a mailto: link, 50 or 60 web browsers would open, but Windows Mail wouldn't. It turns out that the registry value "URL Protocol" was missing from "HKEY_CLASSES_ROOT\mailto". From what I've read this can affect other Windows versions, but I never saw it before Vista.
Slightly annoying bug: beep on listview selection change.
Whenever running older software with a listview, Windows would beep when clicking an a listview item. This also turned out to be a registry issue. It turns out that "HKEY_CURRENT_USER\AppEvents\Schemes\Apps\.Default\CCSelect\.current" value looks empty, but isn't. Simply clearing the value out fixes it. That one keeps re-appearing, so I made a .reg file I can import now and again to fix the issue.
Minor bug: smooth scrolling cannot be disabled in Windows Mail.
Apparently Windows Mail does not honor the system-wide setting, or the IE specific setting for smooth scrolling. I really dislike smooth scrolling, and would like to turn it off.
Overall
I have my memory upgraded to 2GB, and I'm quite happy with Vista. I leave UAC turned on, and it's really never an issue. The machine seems quite snappy, and handles everything I throw at it with ease. Even OpenOffice.org starts up quickly. I don't know how much of that is Vista's "superfetch", or how much the hardware, but either way I like it.