Some of them are not for me- the "big beers only" for example or the "real ale" competitions. That said, I'm NOT into competitions at all. I've entered two- the National, and the Green Bay Rackers comps.
I often have done beerswaps with others here at HBT for feedback, though. The good part of doing a competition is that the feedback is honest. Not that we'd lie, but I feel that some of the reviews here aren't as honest as they would be if we didn't "know" each other.
So, a competition is a great way to get feedback. You get back scoresheets with very helpful hints, and constructive criticism. You might get back, "Great flavor, but a bit thin on mouthfeel. Try mashing at a higher temp (AG) or adding some carapils (extract)" which is very helpful. I recommend entering a few and read the scoresheets.