I'm no expert, but when I took my Wilderness First Aid course a few years back the instructor basically said don't use tourniquets period.
My understanding is this is the direction civilian emergency medicine has gone, with the military only still actively recommending / training / using the technique.
First and foremost, I am not a medical professional.
In December I took a trauma class for firearms instructors. My original post on it is
http://forums.equipped.org/ubbthreads.php?ubb=showflat&Number=211892&page=all (you'll have to scroll down a bit).
We were trained extensively on tourniquets. Loss of blood is the most common killer of people who could otherwise be saved, and tourniquets are extremely effective in stopping bleeding from extremities.