I have.
Hi Johnny --
I've seen a huge jump in my game in the last year and that's with the handicap of not practicing as much as I would have liked b/c of my job. But personally I think people go into a lesson and think they are going to magically come out of it a new player. Two reasons this doesn't happen:
1. They don't go practice their new found knowledge over and over again until they can't do it wrong. Then it comes up in a match and they don't execute it properly and they wonder why.
2. Whenever you change anything about your game, there is small window of time that your game actually gets worse before it gets better b/c you're not a master of these new skills yet and it takes time to become comfortable with them. Some players see this drop in their game as a sign that this new knowledge is wrong and stop implementing the change all together.
Personally I think it's a combination of lessons, practice, tournament play (or any pressure competition) and watching people better than you.
Playing on a Pro-AM tour up here in NY, I play people ALOT better than me all the time and it exposes so many of my weaknesses that I wouldn't see if I were playing someone my own speed or lower. When I don't kick properly, or get out of a rack I should, or play the right safe, or hell, if I break and don't squat the cue ball in the center, these higher players punish me! I see shots I struggle with alot more clearly when I play in these tourneys b/c under the pressure, the shots I'm scared of don't fall in.
I then go to my coach and say, "hey, this shot came up or this was the layout, why didn't this work?" or "I'm struggling with kicking, or stop shots, or ....." You get the picture. Sometimes TR and I will just play racks and after the break, he gives me ball in hand and simply says, "what's your strategy?" Then he can see both visually and verbally what I'm comfortable with, what I'm not comfortable with and usually the stuff I'm not comfortable doing is stuff I just haven't practiced enough or don't know yet.
And the just watching better players ... wow -- so many shots I've learned simply watching the higher players play -- strategies I never would have seen -- safes that never occurred to me.
Sorry if this is more info than you asked for but I kinda don't see this as a simply question. A top pro can give you the tools to get better, but it's up to the player to give those tools the best chance to work for them.