I had a very intense (unbalanced) work ethic after I survived several rounds of layoffs back in 2001. They scared the shit out of me. I spent the next couple years beating my physical and mental health into the ground as I strove to prove my "worth." I was in my early 20's, had just gotten married and bought a house, etc. My sense of security was in shambles.
I acquired a healthy jadedness after I matured a bit and recognized the significance of the following: my boss (my mentor) knew I was doing this to myself and knew I was scared to death of losing my job. As a "company man" himself, he almost encouraged it and never once thought it prudent to tell me it wasn't necessary.
I've always been very good at what I do, so I learned to find my sense of security in that. These days, there is a lot more at stake (larger house, two kids, a healthy obsession or two, etc), but I never worry about it. I just stay on top of my skills and know that if I find myself out of work, I have the skills, experience, and personality to find more. Whenever I start to doubt this, I go out and procure short-term work if for no other reason than to reassert where I stand.
I firmly believe that a job worth doing is a job worth doing right, and I work accordingly. However, I don't kill myself. I don't tolerate unrealistic client expectations and I don't put myself in a position where my work-life balance is threatened. From 8 - 5, I'm prepared to work my ass off - and frequently do. But at 5:01, my life belongs to me.