I never took lessons and I struggle with basic theory. I just got off a good thread with Lalimace about Modes. I got it figured out now but I've been playing for 9 years. You'd expect a guitarist to know these things by now.
I never really took the time to look them up. With a teacher I would have gotten that theory down in no time.
It's also easier to learn the right way first instead of unlearning the bad habits.