I did SAT tutoring for a while with Kaplan. (I was going to do MCAT, but then decided that training for it would be too reminiscent of studying for the thing in the first place). I agree with what a lot of people have said.
1. The pay is just too little when you consider all the time you put in. With tutoring in particular, you have to travel to their apartments. In NYC, that can be an hour each way. Plus, to be a good tutor you have to put way more than the recommended prep time in, which you only get paid like $7 an hour for.
2. It is very difficult to get their raises, especially if you are a part time tutor. I thought I would easily be able to increase my salary, but there are so many stipulations, including the number of "high score guarantee" students per session, near-perfect evaluations, score increases. A lot of this is beyond your control.
3. I did not buy into the Kaplan methods, at least for the SAT. And teaching already overprivileged high schoolers how to outwit the SAT doesn't make me feel like what I'm doing is worthwhile. In addition, you deal with ridiculous expectations about how you are miraculously going to turn someone's lazy C-student into a genius. You probably wouldnt face these issues with MCAT, since most people are already pretty self-motivated, and they don't have their parents call you to complain (I hope).
Overall, I find it doubtful that you would be able to make enough money teaching for Kaplan. In addition, I would be very wary of putting yourself in the position where you HAVE to work for them for a living. There is a reason the turnover rate is so high. Some people love working for Kaplan, but they seem to be in the minority.