I really don't mind if they throw some random celebrities into the mix, as long as it's funny. Also, if they declare, "HEY! It's......", it's also not a big deal to me because half of the time I don't even know who the celebrity is.
However, I must admit the best way to introduce a celebrity is to have them run with the plot, which South Park does very well. Hell, they even did that contest winner thing last week with some random kid who was in the story and had some sparse dialogue. When Kyle asked who the hell he was, he just said he was the contest winner and left. I got a kick out of that one (even though it's been done on Drawn Together, too).
I'm not saying that all guest appearances are good, (because there are some pretty lousy ones), but that they really don't bother me and I usually find them to be pretty funny.