I have a couple of comments to add here. I considered going to ATP to get my MEI, but after a couple of phone conversations with their instructors, I decided not to, as I actually had more multi-engine time than they did and I dont have much. I know that everyone has to start somewhere, but I didnt particularly want to pay for that sort of "professional" training.
If you go, they expect you to have the knowledge part of the work done prior to arrival. There is no way they could pour it all into you in the short time that you will be working with them.
I did a fair amount of research about them before I even talked with them and they do have a pretty good reputation for what they do. Just remember, they are only teaching you how to pass a test.
A couple of the FBOs in my area want you to have trained at the FBO before they will hire you as a CFI. But, there are always exceptions.
A long time FedEx pilot once told me that you should get your licenses done as quickly as possible, because you'll have plenty of time to get your "experience" later.
I dont think the comparison that has been made here between "GA" training at an FBO and "professional" training at a slam dunk school is valid. Each has its strengths and weaknesses.