May I throw in my two cents?
For me personally, and because I am 22, and because I truly enjoy my job (game engine programmer), I tend to put in way more hours than I need to.
Way more hours.
On the order of 80-100hrs/wk. Of coding. (Which is to say, 80-100hrs/wk of actual thinking + typing. Not "time spent getting ready to program" or "time spent psyching myself up to begin working on the problem" etc.)
(Also, I love Vim. I'd likely be massively frustrated with all the programming I was trying to do, if not for ViEmu + Visual Assist.)
And damn, it's fun! The only strain I feel is from lack of sleep.
Also, I think the 'quality' argument is subjective. It's true that if you're physically exhausted, you can't work. But if I'm merely pushing myself, the output of my work is of a similar quality as when I'm living a "balanced" week.
But the insidious thing is, I have a personality where if I try to "balance" myself, I wind up mentally not "getting in the zone" for any of my projects. And for me, it's a little depressing to be in that state, because I am only capable of bug fixes / minor tasks. Nothing architectural.
So, in conclusion, I am certainly not planning on keeping up this lifestyle forever --- I'm just enjoying the 80-100hr weeks while they last. It's a similar feeling to being a kid staying up all night at a friend's house. It's fun!