Pros
Great company reputation. Lots of different lines of business to move between over the course of a career. Pretty good as far as work life balancing, especially in terms of coming up with flexible working arrangements for people with differing needs (e.g, work from home). The stock purchase plan was pretty generous in terms of the discount versus the market. There were always a lot of training opportunities available. There are a lot of smart people especially in highly technical parts of the company, but the business side functions were weaker in comparison. IBM still does offer the opportunity (though no longer the guarantee) of long term tenured employment.
Cons
Career pathing is very nebulous outside of Global Services. Expect a slow, slow progression in your career track even if you are a strong performer. Company is pretty conservative and not very entrepreniurial. Managers weren't always well trained in career coaching and development skills for employees. Although they are clear that 'you own your career', sometimes it felt more like an excuse not to have well defined career tracks instead of really empowering employees. There were not a lot of opportunities to network with other groups in the company which also made it more difficult to take advantage of the size of the company in terms of growth opportunities. Salaries are OK, but don't expect much in the way of incentive compensation or substantially better pay for high performance.