It is only recently that I started seeking out critique groups. I've participated in NaNoWriMo for three years now and been a municiple liaison for my region for the last two of those years. In November of 2011, our meet-ups progressed into forming a critique group called Swindon Free Writers which meets up the first Saturday of every month. It has actually been very helpful for me in showing me where my weaknesses are in my writing and where my strengths are.
I also joined the 12 x 12 in 2012 picture book writing challenge this year, and through the Facebook group, joined a critique group of about 6 people. It has helped me immensely as it showed me just how little I knew about writing picture books and has really helped me to step up my game when it comes to writing picture books. I have learned so much through the challenge and the critique group.
Also, in the Chapter Book Challenge, a lot of us got together in the Facebook group and started sending our stories to one another for critique, ideas and general support. I'm happy to say that one of our members has signed her book series up with an agent now, and, having read her story, I know it is good and I have every belief that we will be seeing her book available to buy on the shelves very soon!
I do believe that, with all of the writing that I am trying to get done, I sometimes take too long to get to the critiques I owe the members of my critique groups. It takes time to give a decent critique of someone's writing. And you need to know enough on the genre they are writing in to be of any help with your critique.
You also need a thick skin. Some of the critiques you get back from people will not be what you want to hear. Most of the time though, I find that my critique partners tell me the things I expect to hear, things that I am already aware I need to work on in the story, so they help me to confirm my own writer-instincts.