Pros
Working at IBM is like getting another master's degree. There is so much opportunity to learn in this company, and in general, the willingness of senior technical leaders to teach (and of managers to support such education) is remarkable. Although organizational politics will sometimes create barriers, the potential of this large, talented organization can be incredible when focused & engaged on a goal. IBM also seems to be one of the leading firms in terms of "walking the talk" on work life balance. While extensive use of work/life integration tools may be seen by some as stepping off the career fast-track, it is widely used, and the level of flexibility in many divisions/organizations is quite high compared to peer firms.
Cons
Bureaucracy is incredibly high. People often compare it to the US Military or other such government bodies. There will be 6 levels of approval for a $250 purchase. There are layers upon layers of executive management. At any given time, at least one other IBM division will be designing a product that competes with yours.
