I think it would be beneficial for most riders to learn from several different people. That way you get different perspectives. Also, it's tough to find an instructor that can teach you the bare beginnings, up to the advanced levels.
The first person I ever took lessons from was my mother. It was a "learn on the fly" experience! Later on, I took lessons from a girl that didn't really know how to ride properly, but taught me the cues. I refined that with another instructor a few years later, although the lessons started to get monotonous, and I could tell that I'd reached her peak of knowledge because even after doing everything correctly, we didn't really move on. I got the feeling I was just another source of her steady lesson income.
I stopped taking lessons for a long time, and just rode. When I finally decided to take some lessons for fun, I started looking around. The barn managers of the barn I boarded at gave lessons, but I avoided them, due to the fact that they did NOT enforce safety of the human or horse. I watched one little girl, in particular, who was born with fetal alcohol syndrome, brush a horse, then go UNDER the horse's belly to brush the other side. They saw my reaction and laughed and said, "Oh! She always does that! It's okay! The horse is dead broke!" All I could think of was, "Yeah, but someday she'll try to do that to one that ISN'T dead broke, and what happens then?"
Another instructor I had heard of was very loud, but really gave no good, clear instruction. I finally narrowed down my options to 2 instructors. It's really just best to see how these people act in and out of the arena. How do they act to their animals? To other people's animals? One instructor (the one I didn't pick) told people to "kick" her puppy "hard" if he tried to jump on them. That told me volumes about her character, and the way that she trained animals in general. Positive reinforcement is always better than negative, in my opinion. I've seen her riders really force a horse into a position that the horse isn't ready for, and as a consequence, the horses have gotten nasty or gone lame.
My instructor does not show, as she does not like the atmosphere. But she went to a very nice horse school on the east coast, and taught there for 3 years after graduating. We focus more on improving the communication between horse and rider, and improving my riding skills. But that's what I've wanted, and what I've asked for. Someone that wants to advance through the levels quickly would not be pleased with her teaching style, as she makes sure you are completely competent before going forward. But improvement and gaining skill takes patience!
I would definitely recommend that you pick out a few random lessons from each instructor you're interested in, and drop in to watch. See how the lessons are structured, how the rider responds to their comments, if their comments are derogatory, how well do they actually instruct, etc.