Big Blue is an excellent Diving Center. I did both, Open Water and Advanced courses, and it was a great experience.
Great instructors, especiallyErnesto was an awesome instructor, professional and at the same time really funny. The accomodation and people staying in Big Blue is another important point to take into account.
Summarizing, if I come again to Koh Tao, I'll come to visit Big Blue again, for sure.