How the hell do I play Gragas? (aside from badly, i mean)

Anyhoo, I've had Gragas for a few months now and I play him off and on, but...

I can never seem to get the hang of him, in Classic *or* Dominion. I've read guides, I've watched videos and streams, but still, nothing.

I'm not sure if it's because I build him as a mage (as opposed to tank or AD or something), but... I'unno. Am I just really bad or missing something?