I grew up with formal music training, including college level music theory.
2 things I can say about it. It can be a source of inspiration, and help you get out of a logjam. On the other hand, at times, it can also be a burden and hindrance, as sometimes I find myself being subconciously limited or rather shackled by it.
Furthermore, one has to understand that most of music theory that's being tought, including multi-part harmonies, is based in medieval/baroque 4 part choral music, and many of the "rules" are designed to help each "voice" stay distinct while working well with others. For example the rule where you can't have parallel 5ths. This is because 5ths (and especially 12ths) sound like another harmonic of a note, and voices lose their individuality when you have them moving in parallel 5ths.
Obviously, a lot of this stuff is simply not applicable to most genres of music today, and in fact throughout history, composers have challenged and expanded upon this kind of stuff.
Where I find music theory useful is making it easy to find different keys and modes that work well together, allowing for more varied and colorful stuff (used wonderfully in Jazz), if so desired.
It just gives you a wider musical vocabulary with which to compose.
