I've used Vista at my mates a few times. His PC only has 768mb ram, which sucks because Vista requires so much memory (2gb or something like that). I don't think I'll be upgrading to Vista for a number of years (if I don't switch OS). It has some nice new features, but I didn't like the 'feel' of it.
I'm sure Microsoft would lose a LOT of users if they did that.