Its very bad for your teeth. In high school I was very active in cheerleading and I ALWAYS drank Gatorade. I went to the dentist my senior year and got an ear full. He told me the sugar content was high and not only for your teeth but something you don't think about is your saliva. To much sugar causes your saliva to get all stringy which is what causes the damage to your teeth. I was urged to choose water over Gatorade but IF I did decided to drink it, only drink it when you are exercising and in between drink water.
In general, sports drinks cause softening of the dentin. Gatorade and Powerade, however, cause significant staining.