The first few seasons up until the movie were absolute comedic brilliance. Seriously, the only other cartoon I can think of that is comparable in it's amount of excellent episodes/quotes/moments/etc. is The Animaniacs. The movie was fantastic as well, being one of the few films based on cartoon/animated franchises I've ever seen that was actually good.
Once the creator left (which was after the movie was released), however, the series took a steep nosedive in quality. While there were a few good moments here and there, from 2005-2008 the show became a disappointing pile of mediocrity, and from 2008 on the show has become nothing more than a festering mockery of what it once was.
Despite all the crap its produced, those few seasons of brilliance from 1999-2004 are so fantastic that they make up for what its become. Still sucks that such an excellent show had to become so terrible, though...