Pros
It's a market leader with lot of opportunities. With over three hundred thousand employees around the globe and core business that spans hardware, software, and services, there are tons of opportunities to learn and grow in just about any area you field you care to pursue. Also, IBM is good at listening to it's employees. It is an organization that encourages autonomy and contribution by it's personnel. This may be due to the proclivity towards hiring independent minded technical people. In fact, you do not need be a sales or management star to excel at IBM. There are many, many individual contributors on a technical plane that are akin to executives with their power and pay.
Cons
It's a behemoth that encourages specialization and has an engineering dominated mindset
