I firmly believe that good writing is good writing. Therefore I believe that a children's book author can critique a mystery manuscript, a romance writer can critique a science fiction manuscript and vice-versa. But a writer in your same genre can add something extra. As a writer in, say the mystery genre, the assumption is they know and read lots of mystery. So, while your friend who writes romance may be able to say "There's a hole in the plot in Chapter 2" or "Your minor characters are a little shallow" someone who writes in your genre can see things others might not. Looking at your writing through the eyes of your genre they can recognize when you're imitating another writer's style, when your main character seems like a rip off on another character, or when you aren't following the format readers in your genre expect.
So find yourself a friend who writes in your genre. But what if you can't? My writers group has people to critique my historical novel but no fellow mystery writers for my cozy mystery. So what's a girl to do? Get yourself to a computer. Oh wait, you're already on a computer! There are plenty of organizations that link genre writers around the country--around the world--together. Here are a few you should check out:
Mystery Writers of America
Sisters in Crime
Romance Writers of America
National Association of Memoir Writers
Science Fiction and Fantasty Association
Thriller Writers Inc. http://www.thrillerwriters.org/