I know some folks have gotten into bad groups, but I can only report positive experiences from mine. It really helped me to have several people who read and write in my genre look at my story and tell me what was working (for them) and what wasn’t. This gave me a fresh look at the story, the characters, the writing in general, and it helped me learn to take criticism. That’s almost as important. I learned that they’re not criticizing me, they’re critiquing my story.
Big difference! I have gotten my share of rejection letters from agents and editors. When I was lucky, they told me why, but so many of them rely on form letters now (or form slips of paper) that I needed another source of feedback.
Another reason a group is a wonderful thing to have is simply for moral support. As fellow warriors in the same trenches, they can offer encouragement. My husband used to refer to my writing group as my support group. And in some ways, it is.
The trick is to find the right group for you. If you are fragile about face-to-face critiquing, try looking for an online group. I was with the Del Rey Online Writing Workshop when it started (I was a beta tester!). I stayed with them for a while once it became the Online Writing Workshop. I didn’t have time for two groups, though, so I kept the face-to-face one, but if you write science fiction, fantasy, or horror and you don’t have a group, you should check them out. There is a yearly fee, but it’s not too bad. With the price of gas these days, I probably spend that in a year going to my group meetings.
And of course, joining a group is a little like joining a gym. It's not enough just to join, you do have to write, too!