I've only used pellets. Just dumped them and by day seven they had broken apart and sunk to the bottom of the fermentor. I've used whole leaf hops in the boil and ended up getting the ball valve stuck.
I just transfered my first IPA to secondary and went with Amarillo pellets for the dry hopping after a long discussion with one of the guys at my LHBS. He was very helpful and described the pros and cons of each.