Working 90 Hours, Paid for 50: Is it Worth it? Are you Rested?

Can you get so sucked into a job that you end up working way too many hours?
What percentage of your time is really productive?
At what point does tiredness take over and you are just walking through the motions?
Why would you kill yourself with a crazy schedule if on top of that you are on salary with no bonus and there is no icing on the cake beyond the 50 hours (per week) mark?