I think there's a difference between knowing theory, and knowing how to apply theory. Knowing the first and not the second can make you sound like a robot, or at very least, very derivative. It's also possible to know theory and be a pretty poor player. Obviously this is verging on reductio ad absurdum (and my theory is shaky and rudimentary at best, I'm not saying I'm amazing at it or anything like that :oops: ), but I know a little bit of music theory and I can't play a note on a trumpet. Learning more music theory won't make me better at the trumpet unless I practise the trumpet as well.
I'm very wary of people who claim that knowing theory killed their creativity- more likely, the theory just showed them that that killer, unique G, C D chord progression they came up with was actually a total cliché. I mean you wouldn't say that learning proper spelling and grammar made you into a bad writer, would you? Or even reading a few classic books to make sure you weren't just ripping them off.
However, I'm also wary of people who act like you can't be a good player or enjoy it without theory. Because that's nonsense too. Aside from anything else, theory is sort of a combination of all the collected music knowledge we have that people were doing instinctively before music theory existed- music doesn't come from the theory, theory comes from the music. (I think. Don't quote me on that. :lol: ) It's also worth pointing out that theory is a suggestion rather than a rule, most of the time.
I think a good quote (I forget who said this... it might have been dimebag but I'm really not sure) is (paraphrased) to learn as much theory as you can, and then forget it. Hopefully it goes into your subconscious, kind of thing, and you'll instinctively know what you're doing.
It also depends on what you're like, of course. If you've tried to "get" music theory and it's just not going in and you're having a perfectly good time without it, I wouldn't worry about it, kind of thing.