I think specialty classes have the potential to offer a diver a great deal, if the instructor puts some thought into the class, and thinks about adding value.
My PPB class consisted of a couple of dives, one where the instructor took weight off and put weight on and I ended up confused, and one where he handed me over to the DM and told him to take me below 60 feet, so I could get comfortable.
The PPB my husband teaches starts with evaluating somebody's equipment, and making sure everything is adjusted correctly, and understanding where all the inflation and deflation options are, and where weights can be put. Then weighting is evaluated, and THEN balance is assessed, and weight is moved around until the diver can float more or less horizontal. The diving portion consists of going to vertical features of the dive site, and doing small ascent and descent and hover exercises (which often aren't described as that, before we dive, in order to keep students from getting anxious and losing their buoyancy because of it
). In addition to the information about what proper weighting and balancing of equipment does to your buoyancy, they get specific practice, and good role models. It's still only two dives, but I think it's worthwhile, and I think we definitely add value to what you could get from reading the book and trying it yourself.
My Fish ID class was a distinctive specialty, and was one of the best classes I have taken (and I've taken a LOT of classes) in terms of increasing my enjoyment of Puget Sound diving.
It's all about what the instructor does with the class. If they do nothing more than what is in the book, you paid them for very little; if they have thought about it and figured out where they can make the experience of working with them ADD to the minimum, you can get a very good deal.