Filler episodes, a lot of the time, are good. They're little glimpses at other scenes not taken from the manga - funny or scary or badass little bites of story. One of my favorite episodes of Rurouni Kenshin, for example, is the one where they ride a train up the coast. It make sense for the period, it shows some fun character interactions (Sanosuke does seem like the type that would be wary of new technology, doesn't he?), and has a simple but entertaining plot. And the aforementioned Goku/Piccolo driving episode - a silly little diversion from the main, serious plot. There's also stinkers, but... if it's just one episode, does it really hurt?
It's filler arcs - long stretches of filler - that AREN'T good. The entire second half of Naruto. The end of Rurouni Kenshin. Bleach's Bount storyline. They're not BAD ideas, in theory - a lot of the time, at least; stuff like Feng Shui Ninja Wars are just painful - but they're stretched and mangled and distract too much from the main plot. If they suck, unlike episodic filler, you need to deal with this suck for weeks while they try and hammer things out.