If you are considering doing your open water certificate in Koh Tao, do it! Big Blue is a great place to introduce you to diving.
I did my open water in late October 2013 and loved it. Luke was a great instructor and put us all at ease. We had a really great dynamic within our group of four, so it felt really relaxed.
After a day practising in the pool, we got to do four dives in two days, and saw some amazing sights. (No turtles or sharks though, alas!)
Accommodation is included (4 bed dorm, very clean but no WiFi) and the Big Blue bar is a good place to hang out.
I have been traveling in Asia for 10 weeks and diving with Big Blue in Koh Tao has been one of my absolute highlights during this trip. Do it!