While I agree somewhat with the point that prep classes can be slow at times, especially for those who score around the mid-160s on a diagnostic, I also think that the value someone takes out of a prep class depends on how they learn.
For example, I am definitely a visual learner -- diagrams, proofs, etc. written out on a board instead of in a book help me learn material better than if only written in a book. I also learn from having someone describe out loud the various logical phrases -- sufficient v. necessary, etc. Perhaps that makes me strange, but it's how I learn.
For me, I scored below 160 on my initial diagnostic, but I got a lot out of my prep class teacher watching him draw diagrams and logical statements up on a board and listening to him explain them, rather than reading the material out of a book. I literally have some of the harder game diagrams that we reviewed in class "burned" into my mind's eye, so that when I come across a similar game on a prep test, I recall the image of a diagram and go from there.
However, once I started PTing in the upper-160s, I stopped going to class. Teacher was otherwise a jerk, and the class moved way too slowly for me for my strong sections, and too fast for my weaker sections. I think if you're consistently PTing in the 165+ range, it's better to just go through the PS bibles and copy sections from PTs to practice the PS concepts. So, for example, if you're going over the PS bible, do the practice problems in the book, but then copy only games sections from like 5-6 PTs and practice bible concepts with those games.
I found this strategy most effective for me. If you are scoring only in the 150s, however, for your PTs, take a course. It may seem like a lot, but it's definitely worth the investment if it helps you bring up your score to one that will make you eligible for schools in a higher tier. Also, if you're a spatial learner, a prep class might be better for you just because you'll see the diagrams up on the board, rather than only in a book.