I find they get me out of ruts.
I definitely agree with that. I was mostly self taught and took some group lessons to learn basics. Then I just spent a lot of time on the web and learning by myself and even though there is so much information on the web, I didn't feel I was progressing as fast as I'd like to. It's not that I didn't practice, but I just didn't know what to practice. Then I started taking lessons with a really good teacher and I noticed my skills improving significantly after just one week. Just make sure you don't get a teacher that doesn't have a plan and just asks you "what do you want to learn". I would sometimes ask my teacher to teach me something and he'd say that I'm not ready for it yet. I believe that that was the correct approach. However, after a few years with him, the rate of improvement decreased and I left. I've been without a teacher for some time now. I should start taking lessons again soon but I feel I'm not in as much of a rut as I was the first time. Now, that I have the basics and then some, I know what I have to work on and I'm aware of the weaknesses in my playing.
Also, there are some mistakes that I used to make that I really don't know how long it would have taken me to fix had I not started taking lessons. They were big mistakes and it would have been very difficult to fix it by reading about it on the web.
my point is, at least at the beginning, lessons are a very good idea..