I had high expectations for Big Blue after reading all the rave reviews, and after visiting there I would say my feelings are mixed. Overall, Big Blue is a great dive school with worldclass instructors and divemasters. I completed my Open Water course with them and really enjoyed it and the instructor was great. But, Big Blue seems like a case of a small dive school that has gotten too big a bit too quick..things were always a bit hectic and my boyfriend and I ended up feeling quite shuffled around when we had a question or complaint and that definitely put a bit of a damper on positive diving experiences. The dorms seem to be well-priced, but I found the private rooms to be bit overpriced. This may just be Koh Tao prices though!! We visited at the busiest time of year, and things were definitely a bit hectic, but I think they could put a little more effort into their service since its grown to be such a big and popular dive school and destination.
For diving courses and fun dives, Big Blue is a solid choice but it may be worth looking at other options too when it comes to accommodation/service. It's also worth mentioning that they really encourage Open Water students to go for an SSI certification, which is excellent and equally as known and reputable as PADI (I did SSI)! But if you are set on PADI, make sure you speak up to the instructors when you arrive.