Pros
The IBM brand is a valuable addition to your resume and the access to large Fortune 500 clients with visible projects is unparalleled. The access to leading edge technology in some ways is nice. There is usually a greater autonomy and less dependence on partners as a young consultant. The people are a good mix of smart technology and business people who are driven but generally approachable compared to Deloitte (Jerks) and Accenture (Engineering Nerds).
Cons
IBM is not disorganized but there is no way it can be considered organized. The performance review and promotion system is one of the most bureaucratic, time consuming, non-value added and just annoying around. Unclear expectations for moving to the next levels; levels spaced very very far apart. A round down system that lowers your performance to your worst review which penalizes you for doing just ok on a first project which you are then penalized for one to two years depending on when you come in during the cycle. Many managers are not helpful in developing you, encouraging you, or even answering e-mails. There is high turnover for consultants here; they chalk this up to the nature of the industry. That may be true but many leave because there is simply no easy way to get promoted other than to leave and come back. Other offers during employment etc. aren't really considered in this system. Two words: Lotus Notes.