I think you can get used to 12 hour shifts, but it still takes up most of your day and leaves you with little time to do anything else. Especially when you are doing 3-5 back to back 12s, it feels like all you are doing is working, eating, sleeping then going back to work.
I prefer 8's and 9's and even 10's are fine. Where I am at we only do occasional 12's on the weekends and they tire you out. It seems like most of the places that do 12's don't really work that many less shifts than we do anyway. The other nice thing about 8's and 9's is that their is a variety of shifts you get to work (morning, evening, or overnight). I prefer evening shifts (5pm-1am) most because I can run errands, get outside and spend time with my wife during the day. With 12's your either working during the day or the night.
Pelivar