The first few seasons were and still are amazing.
The movie is still one of my favorite non-Disney animated movies of all time.
Post-film, though, it just went on a steep downward incline in quality. Now, I can't even bear to watch a single episode made between 2005 and now. They're all just so... dumb, like they're trying and failing to appeal to six year olds' sense of humor.
And these days, the show just looks like crap as well. It used to have a sorta simple appeal to the visuals, now everything is far too detailed, and they're going for the gross-out humor too often, resulting in FAR too much detail in many things. And the focus they're putting on new characters every other episode is dumb as well, since they try to put more detail in their faces to make them an obvious new character. It just annoys me.