I installed Vista SP1 on my PC at home earlier this week. It took an hour to install. I didn't notice any obvious improvements or changes at first. Perhaps it boots a bit faster now, but that kind of thing is very subjective unless you have bothered to time it beforehand.
However, yesterday I found the first real improvement. I had been having problems saving my game in Football Manager 2006. It would take several minutes to save the 200Mb file and would regularly fail to save. Sometimes I would have to attempt to save the game 5 times in a row before it would work. The good news is, that since installing Vista SP1, I have had no errors saving, and the saving is much, much faster (less than 30 seconds now). Its always nice to see some kind of tangible improvement after installing a service pack.