I've been playing guitar for over 15 years, starting when I was 12-13 years old. Those first 2-3 years I took lessons, and I feel it really helps having someone show you some theory (where the notes/scales/chords are) and helping you develop a technique. I then went off lessons for awhile and kind of made my own way. Then went back to lessons for another year or so to do some fine tuning on my now "decent" level of playing. I haven't been to lessons since.
I definitely recommend lessons while starting out for the reasons stated above. I met guys who went self taught from the start and while they developed into very solid players, there was some lack of knowledge that hindered them in some ways. Once you are comfortable and have started your own sound, it's up to you how often you may or may not want to get lessons to help perfect your craft. I will say that no matter how good one gets at guitar they can always benefit from lessons.