Those long shifts just wear me out. Maybe it is just the retail environment that sucks the life out of me but I am usually ready to go home after an 8.5 hr shift!
I have a number of nursing friends who do three 12 hour shifts in a row and they seem to be just fine. 12-14hr shifts are not easy but I think the environment you are working in is going to play a big role in how you handle them.
I prefer longer days because it allows me to keep track of what is going on and better management. A pharmacist working shorter hours can easily give the other shift "their left behind" work, not have to plan ahead, etc.