It really depends.
If you're having trouble approaching the exam, having pacing issues or just have problems with strategy, a good prep class can be worth while.
I took one for the ACT in HS and it really helped with my pacing (got me a good 4 points). This was taught by a faculty member at my school who did know what he was talking about.
I also took prep classes for the MCAT. I didn't find this as helpful, but with the prep-class came access to Kaplan's test bank and other test-prep materials. These were very helpful.
I'm taking a prep-class for step I (sort of), it's more of a guided study course, basically focusing on ways to approach the test and identifying things that I should be focusing on to study. I'm using this to supplement the studying I'm doing at home.
The key point-
A prep class can be a useful supplement to study. If it's good, it will help you approach the test, make you familiar with the test and give you a good strategy which you'll be confident with.
You want to do a lot of practice tests before taking the real deal, this way you'll be familiar with what you'll see on test day and you'll have your pacing down.