Every writer needs beta readers. This (in general) must be someone who is not related to you and not a friend--even if they're always honest about your butt looking fat in those jeans. You're looking for active writers to fill this role, not just people who like to read a lot.
They should always sandwich problems with your work with positive aspects in their critiques. This serves two purposes. We learn much from having the things we do right pointed out, and also a good critique group is supportive by taking your work seriously and by encouraging you.
Generally, the further away from your inner circle of friends and family, the better a working relationship you can create, because, make no mistake, a good critiquer and a writer have a working relationship. If you don't know anyone in your area, see "online writing critique," where you'll find a host of other, free online sources for critiques. I prefer my face time with my critiquers, but I've used Crapometer, a free, collaborative blog. I sometimes submit my short works there to get extra feedback, or when I'm knee deep in novel revisions and need my critiquers' full attention on my book.
The other, less glamorous benefit of critique groups is doing the critiques. It makes you read...well, critically. I generally can tell whether a submitted story has been read by a beta reader. The first clues are clean grammar, clarity of thought, and appropriate spelling. But mostly, it's because the story has creative insight that others just don't have. A good critiquer will often touch on "creative decisions." This means something along the lines of "this character doesn't work for me" or "have you thought of this mechanism" or "this plot point is not plausible based on X and Y." I've had one member of my critique group who has helped me with this extensively, but we as a group are conciously fostering this level of critiquing.
I'm fortunate. I have an excellent, rigorous critique group with whom I have a fabulous working relationship. I'm even more fortunate that they've become my friends, too. There really is no better environment for a writer to be in, among other writer-friends.