Do you need strobothingy flash? Do you need sophisticated manual control? do you need wireless ETTL? do you need to switch to TTL only on a film camera? those are basically the advantages features wise of the 580ex, my answer to all those questions is a resounding NO!
But let's cut to the chase: DO YOU NEED THE EXTRA POWER? that is the only 'real' difference (accepting wireless, maybe), especially as the 430ex is far more advanced than it's predecessor. I shoot exclusively with some type of diffusion or for outdoor fill and the 430 just doesn't cut it powerwise.