I dislike them because they're usually told in a way that is completely different from the main story; character regression, old and already used gags, things of that nature. But sometimes, there's some decent filler that doesn't entirely suck over an extended period of time. The Orange League of the Pokemon series is the only filler worth watching for shows I've seen that comes to mind. There's single episode fillers, but those are usually done in short series, and done superbly (e.g., Samurai Champloo's baseball episode).

Then again, I just downloaded the third and fourth season of Pokemon, along with the first eight movies.