I don't know what's the perfect amount of work every week, but I'm sure anything > 40h is wrong, and you should add commute time if more than 30min/day, otherwise it's a fake figure. In 36-40h/week it's possible to do a lot of things. If you can't often is not because of the time, but because the same people wanting you to work more vaporize your time with meetings, conference calls, emails, phone calls, at a rate th…
If your really dedicated to your project and have no family waiting at home, why should you always leave after 8 hours? Sitting in front of the TV at home isn't necessarily better than an interesting project at work. Will not always be the case, so it's (in my opinion) completely normal to work longer during some months of the year and shorter during others.
For me, flexibility is clearly more importan than working hours. It's great to have a workplace where you can go for a long lunch once in a while or a doctor's appointment without anyone raising questions. It's about being able to make plans. If I know that I can go out for Dinner next Wed on 5pm then it's no problem to work longer on other days. Not being able to plan this sucks, no matter how much you actually work.
But I agree that if the work environment is unproductive, working more is definitely not the solution.