Pros
IBM has a culture of collaboration, integrity, and performance. Individuals are given contribution targets and are expected to achieve them, but working with others is a prerequisite for success. The company often manages by matrix, meaning that you can have objectives from different managers with different objectives; e.g., manage costs and grow revenue. This tension makes for better overall performance, in my opinion. I have worked at firms that belittle their clients when they're off-site and IBM doesn't tend to be that way.
Cons
You can get lost in the matrix if your manager isn't covering for you. It's a big company and you need to have some political savvy as well as the ability to "cold call" within the organization to find the help you require.