Pros
Numerous learning opportunities, both on hands on experience and through a diverse library of online material; if you're smart enough to benefit from it, IBM is a big school that will pay you to learn until you're fired. It's a great place to gather skills and move on. Peers are usually helpful and friendly, you'll very rarely see backstabbing among employees bellow 2nd line management. The opportunity to work with people from around the world. Ethics with business associates and customers - specially government - is a must. You will not see corruption or cutting corners, and you will never, ever be asked to do something shady. No evil shenanigans corporate-wise.
Cons
The "impending doom" feeling of no one being safe, IBM is shrinking at every single month and less (good and well payed) employees makes the management happy. Resource actions take place at all levels and geographies: trust me, you WILL be next - no matter where you are or how well are you performing. Internal climate and engagement are not great, and the management doesn't even pretend to see this as a problem: they openly state their strategy of cutting costs., hiring young and cheap and getting rid of senior expertise with a smile on their faces, while we all frown and despair. There is an incentive of keeping fear of management decisions and strict hierarchy, meaning employees would not dare saying "no" to a management decision. Executives are surrounded by "yes men" and brown noses. This means the company is not as smart as the sum of all resources, but just as smart as the person on top of the tree (and we all know who that is).