Critters is alright, especially for new writers.
strengths:
emphasis on politeness
it's not a big deal to post an unpolished story
weaknesses:
-most of the users are beginners,
-most of the comments are "i hate it" or "i love it" without much explanation of either.
-There's a queue of about a month between posting a story and hearing crits.
-The admin suggest that if you don't finish a story, you shouldn't critique it (counterproductive)
-You have to critique a minimum number of stories to maintain your account
I prefer Baen's Bar (bar.baen.com)
strengths:
-most of the critiques are honest (but not necessarily nice),
-no critiquing requirement to maintain account
-can post more than one story
-Story can get comments immediately after posting
-Counts as submission to major magazine (for a little while longer, until Jim Baen's Universe officially closes the Introducing slot)
-Some of the critiquers are staff members of JBU
weaknesses:
not for the faint of heart (honest criticism often hurts)
PG-13 requirement
Otherwise, it never hurts to ask people you interact with online if they'd like to exchange a critique. Most of my critiques come from and go to people I've met on forums. Don't be offended if they say no--they really might be busy.