Pros
It is a huge company. You can find opportunities on any area, so whatever you like to do, they need it. It may take a while for you to arrange a work position exactly as you want, but this is up to you to show results and find your way inside the company. Don't expect things to come to you: go ahead and open your our doors. One of the best things about IBM is that it has sites everywhere. You can work on the closest office or even at home. I disagree with people who says here that IBM does not pay overtime hours. They do, if you really needs to work overtime. Most people try to earn more money by just working extra hours and than blame IBM cause it does not want to pay for those extras. If they need it, they will ask you to work and will pay you.
Cons
Well.. it is a huge company :-) You have to open your ours doors. Nobody will adopt you there. IBM tries to to everything on IT and sometimes there are no focus. One day they have Network Support Services and the other day they sell everything to AT&T and start saying the future is Mainframe Services. If you want to stay in the company for a logn time you need to watch it closely and be prepared to change areas as the company changes.