I think Louddog has a valid point that lessons can often help to learn various techniques faster and better than trying to stumble onto things on your own. More importantly, lesson can help to avoid learning bad technique (which can be hard to unlearn).
I am basically self taught on several instruments, and while my chops have always been good enough to get me plenty of gigs and a fair amount of session work, I always have certain limitations. When I take lessons, I am able to find things I can do better, which improves my playing. However, it was sometimes awkward to unlearn bad technique that I had taught myself.
The key is to make sure you know what you want to learn and find the right person. I don't take lessons every week (never have). But if I want to learn a specific thing, I'll find someone who I think can teach me - schedule a lesson. After a period of practise (say a month) I'll go back and see if the teacher wants me to tweak something. There are some people who only gave me 1 or 2 lessons - cause that's all I needed from them.
Having been a gigging musician for almost 40 years, I've known many players. At some point, any player (if they are serious about thier instrument) will seek out people to leran from. No matter how good we think we are (self taught or otherwise), there are always people who can teach us to be better.
The best players I've worked with were people who had some formal learning and a whole lot of self taught (mostly from playing with other people).
Johnrg - you mention that you are too far in the country for lessons. Maybe you can't take lesons every week - but perhaps you could drive into the nearest city once a month for a lesson from someone. Naturally, you can still work with books or videos on your own in addition to a lesson here and there.
I have a drummer that comes to see me maybe twice a year. He shows me what he's been working on and tells me what he is trying to improve. I watch him play and then offer a suggestion here and there (in my case I do it for free, cause he's a young guy with a real love of drums and he bought a couple of sets from me over the years, so I try to help him progress).
I know guys who literally fly into places like LA or New York once a year to take lessons with some name players. Often one hour with the right person can be more help than 100 lessons with the wrong person.
Candidly, if you've been playing for 10-15 years and you feel you've "hit a wall" a book or video may not be enough, some one on one may be better.
