I think there are two keys to 12 hour shifts:
1) You need to have a rotation that works. The "Pittman" schedule or a fixed 3/4 or 4/4 schedule would be my preference.;
2) You need to have the right amount of call volume. If your department runs balls to the wall constantly, the longer shifts and potential to have paperwork OT will kill you. If your department is slow for long periods of time, the boredom will kill you.
I have worked twelves in a three on / four off schedule for almost five years now. We are not a busy department by anybody's standards. When I was a supervisor and more involved in training, I loved it because I could get a few hours of office work in reviewing reports, doing research, developing training materials, etc. and still get several hours of patrol. Now that I'm a slick-sleeve again I find myself getting bored and watching the clock tick away more.
"Logic is rarely the engine that propels a police department forward."
-David Simon in "Homicide"