I too would highly recommend Randy's Pool School.
It's been a couple of years since I took the course from Randy and Carl in San Diego, and I have never regretted it, in fact quite the opposite (however, I do wish that our geographic location at the bottom of the left coast didn't make it so difficult to get on Randy's schedule of classes

)
I would say that everyone will see some kind of improvement in some aspect of their game right away after taking the class, but what has been important to me is the long term benefit that I have received over time as I continue to employ and work on what was taught in the class.
I didn't know at the time what a mess my game was in when I first went into the class. All I knew is that I wanted some instruction so that I could identify parts of my game to improve on and to learn how to do it; and that's exactly what I got.
I can't tell you all of the areas of my game that I have worked on to improve during and after the class; pre-shot routine, visualization, stance, bridge (and length), grip, SPFF (don't forget the "freeze" at the end), speed control, eye patterns, the dreaded cowboy lasso pull-back on the draw stroke... Anyway, I corrected many of these areas right away. But I want to point out that the "take away" from the class gives you a personal road map to continue to work on your game in the future. And that's where you'll see the BIG payoff.
I continue to work on drills that I learned in this course, and I occasionally take out and leaf through the training material from the course (I even noticed that I had the Cue-Tech book right by my bedside before I came in to work today).
People in the league that I play in have commented that my game has really improved over time. My game is now light years ahead of where I was when I first walked in to Randy's Pool School, and I am still seeing improvements/progress to this day.