Straight up fireball chucking is just asking to be back-stabbed.

The most effective use of pyro in PvP I've found is Surprise Pyro! When I'm my DEX/Pyro guy, I wait until the other player gets into a spot where maneuverability is very low and then, boom, Fire Tempest. For maximum hilarity, try it on the rafters of the painting hall in Anor Londo. It's also useful in areas with lots of cover - step behind a pillar and as your opponent tries to strafe or circle around it - boom, Chaos Fire Storm.

Of course, you'll want to have a good melee set-up for the times you can't pyro (most times). My Lifehunt Scythe covers that.