I lived on Ko Tao for four months. The diving is crap, honestly. It's a great place for cheap diving and cheap courses, and if you're looking to rack up c-cards, it's good. There are a couple of okay dives, Chumpon Pinnacle and the daytrip to Sail Rock, but nothing to get excited about unless you see a whale shark, but the odds are not too high unless you're there for a long time and do a lot of dives. If you're young and enjoy the bungalow-style vacation on the beach, it's pretty nice and mainly a backpacker destination. There are some expensive resorts and also many cheap bungalows. Keep in mind a lot of places only have electricity at night and ask about it if it's going to bother you. There's nothing to do but dive and party though and the island has an enormous ex-pat population, so don't think you're going to see any Thai culture. If you can, save your money and go to the Similans on the other coast instead! They are really beautiful with great vis and have some nice cheap liveaboards. However, the season is winding down there and you'll have to wait until October before it kicks off again.