I stayed on Poipu beach, at the Kiahuna resort, which is a condo rental place. it's actually directly adjacent to the Marriot (in fact, I even picked up the wireless signal from the marriot). Poipu beach is very nice, and is a great middle ground between the two ends of the island. Very short trip to Waimea, for instance.
I got some good snorkeling in on Poipu beach, too.