WOW. 9 pages of comments. I can't see taking the time to read all of that but I've got a pretty good idea what it says.
I'll relate my experience as a consumer of lessons, group, camp, and private. I returned to skiing 15 years ago very out of shape with poor technique. I thought I was a better skier than I was, but when I attempted to ski off-piste terrain, it was apparent to even me that my skill was inadequate for the task. I had the benefit of skiing with a private instructor through a couple of executive conferences I attended, and I'll say that the effect of private lessons wasn't that useful at that point in time for me.
Copper Mountain had a program (sadly discontinued years ago) called the Sunday Sensation: unlimited group lessons for a ridiculous price (around $200). My buddy and I signed up. What we got was consistent coaching on our problem areas. I started to make real progress as I had someone who was taking my skiing apart, focusing on the big picture, and steadily putting it back together. This was a big help.
I went to Steep and Deep camp at JH. Nice time, but it also suffered from the lack of a consistent diagnosis, split priorities (skiing the terrain at JH and trying to improve your skiing -- sometimes you need to just focus on improving your skiing and forget the terrain), and mixed groups. I started taking the EpicSki Academies, and the learning there was incredible. It really makes a difference having a world-class instructor with the ability to focus on progressing your skiing in 4 days. Too bad they aren't offered any longer.
All along, I'd been eyeing private lessons, but they were soooooooo expensive. Even though I'm fortunate to have the means to afford them, I just didn't believe there could be the payoff to them.
After a really impactful ESA, I decided to take the plunge and do privates with my ESA coach, Jim Schanzenbaker at Aspen. At the time, Schanzy was on the PSIA national demo team. The 4 days I spent with him were so transformative to my skiing that I've gone back repeatedly to ski with him. I've also found great value in privates with a few other highly qualified coaches.
Here's what I've learned about the subject:
1. It matters where you are in your skiing. In my case, taking privates when I was a lousy intermediate skier probably wouldn't have yielded value for money -- that payoff was in good group instruction and in good camps.
2. The instructor matters. Any Tom, Dick or Harry is unlikely to cut it. You need an instructor who is well qualified and relates to you. If you are a feeler and your instructor is a thinker, it's probably not going to be a good payoff.
3. A "tune-up" lesson is unlikely to yield much in the way of results. Learning comes through repetition. Practice makes permanent, perfect practice makes perfection. If you get half the idea from one lesson, how are you going to get the rest of it? You need to realize that improvement is a process, not an event.
4. At some point, private instruction has a huge payoff. But don't do it too soon or without a plan to repeat it. And you probably need to spend a lot of time identifying the individual who will become not your instructor, but your coach.
My $.02. Which is about what it's worth.