At least they shouldn't be. Okay, they can be very nice people and folks that when you actually see them face to face you can sit down and have lunch with and swap stories. But first and foremost, they should be ruthless picker-aparters of your manuscript. People who have no problem slashing your writing with great swaths of red ink. Who can tell you when something is not working or when a paragraph just plain stinks. Folks who can tell you when your main character is unlikeable and when it would be better to just hit delete and start over.
People who read your work and gush over how lovely it all is aren't critique partners - they are friends and/or loved ones who are afraid to hurt your feelings. Don't get me wrong, good critique partners know how to sandwich critical comments in between saying that they liked this description and that this sentence was nice. People who just say nothing but mean things are just well...mean.
In truth, all of the people in both of my critique groups are nice people. Out of the eight people I swap work with, I've only met one of them, and we had a great time, which left me wishing she didn't live halfway across the country. One of my most ruthless adverb annihilators lives all the way in Italy. The advent of the Internet and the email critique group has helped in a lot of ways - mainly that it's a lot easier to be honest when you're not sitting next to the person and don't have to look them in the eyes. And I appreciate that.
On this date: In 1967, Aretha Franklin hits number one with "Respect".