Pros
IBM is a large company with many opportunities to have and hold many different positions through a long career. Opportunities from manufacturing to sales and everything in between exist. Services, technical, admin, staff, teaching, HR, sales, graphic design, TV and Video production, food service, etc. There are few companies where you can have this kind of exposure to many different positions. Large companies offer more stability, excellent benefits, and a diverse work environment.
Cons
It's a big company. It's easy to get lost. Salary growth can be stagnant. Without establishing a network at the company, you will get lost. Many IBM workers are mobile workers and a sense of community can be lost if you aren't active in establishing it for yourself. This isn't the IBM of 25 years ago where Respect for the Individual was paramount and employee loyalty fueled its success. Lifelong employment at IBM is no longer guaranteed no matter how hard you work.