Normally, students start out with a third class medical. It doubles as a student pilot certificate. Then as you become Commercial/CFI you normally get either a 2nd or 1st class, which are usually more expensive. The doctor who I go to makes it pretty quick and painless. Quick eye exam, blood pressure reading, checks your pulse... then it's off to his desk to fill out the paperwork. Nothing major. And if you're over 41 (I think.. correct me if I'm wrong) you need an EKG.