Let's go now, why wait.....

randyg
Randy, you can't go! I've only been to the intermediate class twice. I need to do that a time or two more and hit up the expert class a few times!
In all seriousness, guys, I've been to a couple of RandyG's classes taught with Carl Oswalt, along with a handful of buddies from the Austin area. A few of the students that went with us never really improved any more than their normal gameplay, because as soon as they got home, they went back to their old habits. Any of us that stuck with it improved, and want to give Randy more money, even to go back to retake the SAME CLASS, sometimes 3 or 4 times.
I know that's anecdotal evidence, but it's true. My game has improved. Oh, and Randy did include the lesson on how to play at a pro level. I'm still not there, because I haven't decided to sell my house, leave my wife and kids, quit my job, and play pool 8 hours a day, 7 days a week.
If possible, I do recommend taking a class with a group of friends. I think I've held onto the mechanics far better because I can say, "hey, Andy, pay attention to the angle of my arm, or to my pre-shot routine, or whatever" while I'm shooting a league match, and he knows exactly what to critique. It also helps when we're coaching each other in league or trying to teach each other things, because he quantifies the speed of a shot. Instead of having to think that my "medium speed" is a lot softer than your "medium speed", with his training, I know that my "3-speed" is the same as your "3-speed".
Really though, to whoever said Pool School should be measured by how many pros are churned out by an instructor, that argument doesn't work. Pool school alone doesn't make you a pro. It teaches you the fundamentals of how to play the game properly, and how to evaluate yourself periodically to get your game off of a stagnant plateau. You'll never be a pro without the practice and drilling to instill those fundamentals into muscle memory.
RandyG lets you retake a class for a significant discount, and I feel that it's worth it solely for the benefit of having a well-versed instructor go over videos of my stroke to see how well I've stayed in form, and give me tips to get back to perfect form. The added content each retake is an added bonus (Instructors learn too -- their classes always evolve).
Sorry for the long post, and always be on the lookout for the Murphys. --elint
[EDIT: I'd like to add that my experience with Pool School has been with RandyG's school in Dallas. I'm not trying to sell for RandyG -- I've heard from the boards, and I believe from RandyG himself (don't quote me on that), that other SPF instructors across the nation have comparable programs. If you're not in the Dallas area and RandyG's not coming to your area, there are probably still some very accessible SPF instructors around, but I can only vouch for RandyG, as he's the only pool school instructor I've had]