Pros
IBM's salary and benefits relatively speaking are better than average despite the annual increases in employee contributions. IBM has well defined processes in the various parts of the business which enables employees young in their career to learn good skills. To those individuals new to IBM, stay long enough to obtain the experience the job offers, then move on as IBM isn't good for you long term.
Cons
IBM has lost its shine. I am certain that Thomas Watson continues to turn in his grave as to what has happened to his company. IBM has long moved away from gaining new ideas and starting businesses from scratch to one of acquisitions. As products move through their life cycle, the work is outsourced to places like India where IBM can reduce its labor cost. The net affect is that the employees are discarded as the products they have built and supported are no longer generating the revenue they once were. New opportunities exist only in locations where companies that were acquired are located. Unless you have experience with the products from the companies acquired and are willing to relocate, your future at IBM is limited. IBM is a pro at reducing its labor through layoffs while minimizing the publicity of downsizing and outsourcing. Once you see the death spiral begin at your site (especially sites that have been around a long time), be proactive and find a new opportunity elsewhere. It may seem daunting at first, especially the longer you have stayed at IBM, but in the long run you will be better off (and much happier too).