Nice for employees but most succesful Hackers/entrepreneurs I know work 6-7 days a week. Availability is often key rather than actual total hours worked.
I work 4 day weeks with Wednesday's off, but I'm flexible. If I need to do something on a Wednesday I'll have a different day off instead, if it all hits the fan at 9pm on saturday night, I'll fix it (or one of my colleagues will), If I need to do a 70 hour week then I will (with the appropriate time-and-a-half for the extra 30 hours worked)
Unless I have a big a chunk of equity I'm working for a paycheck, not the company. I know the sacrifices entrepreneurs make: no thank you. Unless you want communism where everyone has equity.