We spent 2 weeks at Hayman in 1999 as part of our honeymoon, and loved it. If you are big on scuba diving, its not the best choice, as others (Lizard, Dunk, ...) are right on the main reef, while diving at Hayman means a boat ride first.
Neither of us dives, so for us it was perfect. We also liked the fact that it was a bigger resort (probably 100-200 rooms, I'm guessing), so there were more choices and activities compared to a 10-12 bungalow island. At the same time, since it is the only thing on the island its not hard to get as much privacy as you want.

We'll definitely go back, once our kids are old enough to leave with relatives or boarding schools for a couple of weeks :-)

Bob