I’ve installed Vista SP1 on several machines. Takes ages, but otherwise it’s been without incident.
This does not dramatically improve Vista (in my experience); but then again, it wasn’t that bad before. It does seem to speed up Explorer and zip extraction. It tames UAC slightly – some operations that used to require several prompts now only require one. Otherwise, I haven’t noticed much change, though I’m aware that it includes numerous small updates.
What I do find interesting is that Server 2008, which has the same core as Vista SP1, is delightfully smooth in comparison to Vista. Just don’t ever install the Desktop Experience on 2008 – this is a separate feature that is off by default – or whatever it is that makes Vista still somewhat prone to sitting and thinking when you want to get on with your work.