Writing Contests

Obviously, I'm not at the point yet where I will be entering any writing contests, but since, at some point, I would like to enter one (or two or three), I'd like to know how beneficial they are to a writer. Besides the fact that entering writing contests could give you the chance to have your work read by editors and, therefore, possibly get you a partial or even a full request for your manuscript, how do they help further your writing career?

I've read on several agents' blogs that if you include the fact that you won so-and-so contest in your query letter it does nothing to sway their decision one way or the other about you or your manuscript, and that a lot of agents just plain don't care if you've won contests before, so how do they really help you? Do you get detailed comments on your contest entry about your writing or do you just get a standard form rejection? How do they work? Which contests are the best to enter if you're writing YA books? Should you wait to enter contests until your manuscript is polished and as perfect as it's going to get, or is it okay to enter before that point (but not before you have a completed manuscript) in the hopes of getting comments and critiques about what you could do differently?

Like many other areas of writing and publishing, this is something I'm going to have to do some research on. However, if anyone can answer some of these questions for me, it would save me the confusion of reading varying information about the subject elsewhere.

On another note, I have surpassed the 50% point of my first draft. My words remaining total is now less than my words written total. I am struggling a bit with where to go with this last half of my book, but I'm getting there I guess. I'm still within my finishing goal date so I must be doing better than I think. At least, I hope so.