If you just want to say that you liked something, your comments are appreciated, but if you can, take a few extra minutes to suggest something the author could improve upon. None of us are perfect. Let's elevate the quality of feedback on this list and not just help each other continue to write, but help each other continue to write better stuff. I want to see all your names on the shelves at Borders. The value of the latter is often overlooked by beginning writers.
Critiques are due within a week.
Most pieces get 15-20 critiques. I highly recommend checking the general information on this site, even if you don't join, even if you don't write in these genres. They have examples of the kinds of critiques that are given in the group and a really handy resource page. This site contains some wonderful tips on how to approach a piece for critiquing, and includes a checklist for approaching a piece of fiction, covering all the major elements including character, plot, setting, etc to style and format. She also has a brief list of recommended reading for writers of fiction and science fiction.