Pros
IBM, being the tech giant it is, is a great place to get your hands on some pretty impressive hardware infrastructure, and learn about many technical advancements that IBM has a consistent and significant contribution to. As an intern, I was happy to go to huge labs, set up servers, learn about many different storage and networking technologies, and deepen my knowledge of large scale server systems.
Cons
IBM is at the same time a very old, conservative, and slow moving company. You don't hear IBM go crazy with new ideas everyday, because they don't do that. Decisions are made only after going through a maze of hierarchy and bureaucracy and you, as a regular employee, constitute an extremely tiny piece of this huge company. If you do not go up the corporate ladder (which is a pretty hard thing to do at IBM), then you are very much replaceable by just about anybody else. You may feel that at an entry level job, you are small, and--to avoid saying unimportant--of little importance to the overall being of the company. I guess IBM makes the most sense to people who are extremely research oriented, or would love to have a stable job to stick to for years and decades.