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?
Maybe it is time to hire an 'agent' who will stop the nonsense right there and get you a better deal.
I was pondering all this after noticing how a recent story I wrote on sleep deprivation echoed with a lot of people.
Where do you draw the line personally?
How do you set the boundaries?

Related: Sleep Deprived Anyone? Why I should sleep more?