There are a lot of writers out there, professional and otherwise, who don't use critique groups. Until recently, I was one of them. The reason why I didn't use them was because I had had some bad experiences with critique groups in the past. These problems, which seem to be universal, come in various forms:
1. the Nice Group - this group only says nice things about your work. Groups like this are like mutual masturbation - they are better than going it alone, and they're satisfying in the moment, but ultimately you're left with the feeling that there's something more fulfilling out there.
2. the Court - this group revolves around one person. Either by force or by acquiescence of the other members, this group revolves around one writer, and all other submissions are compared to the sovereign. These are boring ego-fests, and a waste of time.
3. the Social Group - this group exists only so its members have an excuse to get out of the house and hang out with adults once a month. They are heaps of fun, and worth attending if you have the time, but don't expect them to improve your writing. Sometimes you won't even talk about writing.
The critique group that I was fortunate enough to stumble into has none of these faults. While everyone is very nice and friendly, they also aren't shy about saying "um ... yeah. This part here is not working for me." Or, "what about doing it this way?" They care about the story, and they are willing to speak up to protect it. This is valuable.
I'm going to sound like a little bit of a jerk for a second, but bear with me - I don't need a group for the little things. Grammar, punctuation, spelling? I got it. Dialogue, scene, plot? No, no, I can pretty much handle it. But the big things, the things that you don't see until you're way down the primrose path, like "why is your main character doing that when just last chapter she did this?" And "you've been setting up a big conflict here, so where the hell is it?" and "why did that minor character take on such a big part here?" - these are all the types of things that I can't see in my own writing because I'm too close to the story.
Now, sure, I could wait until I have some distance from the story and pick it up fresh, and letting a story lie is a part of my revision process definitely, but in my group, the readers all have good eyes and when they bring these things to my attention, and that means I don't have to wait two or three or four weeks to do it myself, if I would even see it in the first place. They are invaluable.