I've always found South Park funny, but there have been some pretty crappy episodes. Definitely liked the Warcraft, Scientology, and "Super Adventure Club" episodes. But it's always like they have one or two good episodes, then they add something utterly pointless and crappy. The Towley episodes are usually pretty lame. But then, you have some great ones, like when Cartman goes to the future to get the Wii, only to find sentient otters and the like. Then, they make prank calls to the past as a form of entertainment in the future. Great stuff. I don't think the Family Guy episode was too bad. I think they made som pretty true criticisms of Family Guy's jokes and the like. Most of late Season 4/5 was pretty random crap, not even tied to a central storyline.
Also, SP has a very unique creation cycle -- the latest episode is made in the week prior to airing. So they literally make the entire episode in a week, right before it's due. In a way, it's the ultimate form of procrastination, and a lot of stuff is hit/miss. But I feel like they do a pretty good job pointing out the funny parts and less-known details, tied in with cultural references. I particularly liked the "Global Warming" episode where they made fun of the flood and made references to Hurricane Katrina in New Orleans.
--Terin