Pros
It's one of the biggest companies in the world, so you can try out a lot of different career paths without having to start over. IBM develops many cutting edge technologies, and has high name-brand value around the world as a reliable, powerful company. Stability. Almost nobody can really offer a secure career for life anymore, but IBM comes pretty close, mainly due to its size.
Cons
It's one of the biggest companies in the world, so it can move slow at times. Even though they make some of the most dynamic, cutting-edge technologies in the world, the senior management is trying to do it as cheaply as possible. This means that bonuses, raises, and hiring are low, and outsourcing to labs outside the US is on the rise. The name brand has a good reputation, but certainly isn't as sexy as younger, smaller, more nimble firms, such as Microsoft and Google.