Pros
At IBM you work with some of the smartest people in the world. It still makes me proud to have been associated with an organization that has the reputation for excellence that IBM has. Smart people, great name, and in addition, IBM is so large that their are a glut of resources at your fingertips. So much so, that sometimes you get so used to having so many resources that you become a little spoiled. There is a department or a person for every possible business, sales, or service function. The benefits are also world class. I can remember when I started paying about $34.00 per paycheck to cover my entire family and didn't realize how inexpensive that was.
Cons
IBM is very large, but the downside to that is how long takes to do anything. The red tape is legendary. It takes a long time for an oceanliner to turn around. Promotions can be slow. Progress of any kind takes time and people like to stick to the way things have always been done. If you have a new idea, or process, you have to move heaven and earth to get it implemented. Management can also promote on who you know, not what you know. But this is typical corporate america. It goes without saying that the level of political intrigue is dense at IBM. Kissing bottom is taken to an art form and there are more angles on everything you say and do than a billiards game.