Pros
IBM has that aura of being a great place to work. The near collapse of the company during the 1980's tarnished that image, and experience folks understand the realities of both the luster and the rust. In a company of 300,000 I would be foolish to try and depict the overall company "image", however with more than seven years of experience in the software sales segment of the business I saw some general (negitive) trends. In order to reduce operating costs a large percentage of the employees work from home. Many teams are comprised of staff scattered around the globe, which makes building work relationship more difficult. Conversely this structure provides a lot of schedule flexibility.
Cons
In looking at any position within IBM one must evaluate whether the position has requirements to exist where ever you are located. Ongoing cost cutting efforts may well eliminate your position or ship the position offshore (in the long term). There are annual efforts to reduce the bottom n% (5-15%) of low performers acrosss the company. If you are in a team of superstars, the team will loose their assigned percentage - regardless of the impact. In order to achieve continued growth, the company buys a lot of smaller companies. If you are working in an area of the business that has been enhanced throough a purchase, you risk being cast aside once the new venture comes on-line.