So, a little while ago, I won a 1,000-word critique from Michelle Merrill. She sent it back with all sorts of comments and suggestions. She made one comment (which I'm sure she didn't think would mean anything) that really got me thinking and pointed out a major flaw in my subsequent chapters (and it was actually a positive comment).
Now, I should clarify the difference between alpha-readers/critique partners and beta-readers (which the guys over at Writing Excuses helped me understand, so go visit them here). Alpha readers (or critique partners, whichever term you prefer) are the people you send bits and parts to during your writing process; they help you keep your story moving in the direction you want to go. They help you identify problems so you can fix them (without having to rewrite your entire book). They are the ones who can tell what you are trying to do (and don't try and get you to do what they would do) and help do it better. These are the people you want it a writing group.
Beta readers are the people you send your book to once it's finished (I use 'finished' loosely). They provide you with general feedback to the story as a whole. They give you substantive feed back on plot and character and writing, based on a comprehensive evaluation of the story. Just about anyone can be a beta reader, as long as they know how to explain why they think what they do.
Alpha readers are going to be infinitely more helpful than beta readers, but they're a lot harder to find. And, unfortunately, the only way to find them is trial and error. So, if you meet a fellow writer that you like, ask him/her to join your writing group, or at least to exchange manuscripts (like I said before, usually only a chapter or two at a time), and see if you are a good match.
So, do you have a critique partner? How did you find him/her? How has your CP helped you?