I attempted to install the Release Candidate of Vista Service Pack One.
I've been running Vista (non-Aero) on an old Dell laptop. It has been slow but stable. Mostly usable.
After the service pack allegedly reverts changes (I've tried twice) Vista doesn't do anything more annoying than tell me there is a new service pack update to install. There is no loss of data or functionality. No new error messages.
But it's pretty disappointing. The Release Candidate for XP SP3 worked like a charm on an even older laptop and Windows XP seems to perform as well as it always has, if not better.
At this point, the Vista laptop is looking like an excellent candidate for conversion to Ubuntu.