Pros
Everyone says the benefits are great, but if you don't have a family, then who really cares? I just want to go to the doctor when I'm sick or the dentist so my teeth don't get jacked up. The best reason is really the flexibility of the managers. Most managers don't care if you need to take time off last minute, want to work from home, or need to leave early. It's also nice to have access to free education through the Harvard Extension school and also through the University. There are also some good attempts at "practical" training through the Center for Workplace Development. Plus, if you're a member of the union, you get a raise (about 5%) every year. There are also a LOT of hotties working at HBS.
Cons
It can be a little stiff in the upper levels of management, and there's a culture of "we have it the worst" in most units: i.e. most offices love to think that no one appreciates them and that they get no respect. The other downside is that you can freeze your ass off in the winter if you take public transportation because there are big gaps between the bus stops and the tunnels.