I have been talking to tons of people who are either presently working at McKinsey, Bain, or BCG or are alumni of the three. What I've figured out is the following:
1)At McK you are pretty much guaranteed to travel M-Th and will work long hours on those days. Weekends are well protected. The big advantage to McK seems to be an enormous ability to control the kind of work you do, particularly if you come in as an APD. Everyone I've spoken to said they only worked on studies that interested them and were never forced to do otherwise. As a result they did a ton of travel but have established impressive expertise in certain areas.
2)At Bain, people seem to rarely travel. One friend who has been there for 7 years said he NEVER did the M-Th travel gig and usually only did a day or two of travel at the beginning and end of a project. He loved the people and work but had considerably less control over the work he did, particularly in the first several years.
3) BCG seems to do 2-3 days of travel a week and is similar to Bain in that you have less control over the projects you work on. It seems to have less of a true culture than Bain and McK, but that might just be a reflection of the people I spoke to there.