I don't know how it is now, but two years ago in Ko Samui the 3G internet was unusable. For example I never managed to log in to Facebook with it. It would work for a few seconds, then the connection would disappear for a minute and then work for a few seconds again. We had more luck getting a very cheap cabin next to a very expensive resort and use their wifi. Forget the internets, bring a few good books and enjoy your stay =)