I think classes can be very helpful. I've read a lot of books and magazines, and have read a lot of posts on internet boards like BN. While I think I have a pretty good sense of general knowledge, it's the practical knowledge that I find myself lacking, which is where I think classes can be of help.
For example, I took a Boon repotting workshop through my club earlier this spring which was immensely helpful. A couple of hours watching a master explaining at each step how to go about repotting, and with all types of trees, was more instructive than all the chapters on repotting that I could have read. For me, even just learning how to properly wire in drainage screens and wire your tree into the pot were worth the price of admission.
Then again, it probably depends on the quality of the instructor you are thinking of working with, the subject matter, etc.