If you don't have a diffuser get this. I've had a 580ex a while and just used bounce with odd bits of card occasionally. Once I bought this images are improved dramatically and it's not as imposing as other diffusers that you can buy. My only problem is it doesn't store well (as it adds an inch to your flash's length).
If you want something grander like the lightsphere etc, then fair enough, but for my needs of something a little subtler (OK an SLR and flash aren't too subtle, but every little helps), this was perfect.
I'd also recommend you buy at least a CTO (orange) gel for a better effect in incandecent light.