PC World tests the final code for Vista Service Pack 1 (SP1). The results are generally favorable, though the performance improvements are not as great as many might wish. The article is an interesting read, though I would wait on more tests by other organizations and researchers before drawing any firm conclusions. It's one of those "your mileage may vary" deals, to be sure and even PC World calls their results "informal" and "preliminary".
Still, it's well worth reading. I enjoyed it, too.