Worked at 2 different firms in DC. The first (which I worked at for about 3 years) was more BigLawish than the second.

byujag's description above is pretty accurate.

I realized very early on that I was doing it solely for the resume experience and not to make partner. Accordingly, I only worked "late" (which, at BigLaw, means after 9 or 10 pm) or over weekends when partners forced me to (which happened much more at my first firm than at my second) and never voluntarily.

The lifestyle can be very hard on families and, in that regard, I wrote a post about a month ago about how very few LDS attorneys I knew back in my BigLaw days managed to make partner while keeping their marriages intact and staying active in the church.

I'll share one ancedotal experience. After I'd been at my first firm for a few weeks, I was invited to a Friday evening happy hour. I arrived around 6 pm and stayed to about 7 pm. As I walked out with a few other young associates, I assumed they would all be heading toward the Metro station or parking garage to go home. Instead, most went back to the office. That was the moment I knew I'd never become a BigLaw partner.

(FWIW, I've been in-house for the past 10 years or so).