To my knowledge, Universal gets almost all of its attendance during the summer and tend to have a pretty slow fall and winter. When I was there in August a few years ago there was about a 2-3 hour wait for the Hulk at IOA and even with there free fast pass-like thing that they had then it was 1 1/2 hour wait for Deuling Dragons. The waits are not really from the fact that there were a lot of people there, just that the paths aren't really that big there and there are really only about 5 or 6 attractions worth caring about. The Studios side has a lot of outdated things and attractions like Shrek 3d that blatantly make fun of Disney, however rides like The Mummy, Twister, Men in Black, and Back to the Future do get large crowds as far as wait time goes.
Last time I was there it was spring '06 and they had a decent amount of people in the park nothing was like that one Summer I went though. I've got an Annual Pass for there, just because if I go there twice during the year it pretty much pays for itself. Even on a slow day I've never managed to park anywhere near the park in the parking garages. Finally beware their Express Pass, it changes price depending on the season I've seen it almost at $70-$80 one time last spring, so I wouldn't count on being able to use it. I mean unless you like to spend money or it's at a really good price that day for whatever reason. If the lines are longer use Single Rider Lines wherever you can or meet a very nice lady with Medical problems that has a handicap pass to allow her and family onto rides, that's what my friends and I did one time.