The known climbing on Koh Tao probably isn't developed enough to warrant a visit on it's own. However there is a lot of bouldering to be done and would be a great place to combine a beach/diving holiday with some bouldering. If you're visiting Thailand and want to get away from the usual climbing areas then it may be a good option.
The closest areas to the main town Sai Ree are within easy walking distance. The boulders are extremely coarse granite with large sharp crystals (bring extra skin).
Inland, mosquitos can be a big problem depending on the time of day. Early morning/late evening are prime times for the mozzies. If the mosquitos are out in force there is plenty to do for example; diving, wake boarding, cliff jumping etc.
Lots more information on climbing and other activities is available from this website. The shop is based on the Sai Ree beach. Good Time Adventures
Hot and humid.