Vantaggi
IBM is a recognized name that will help you get your next job. In some parts of the company, IBM may be an industry leader (this assertion is not true for the consulting business).
Svantaggi
IBM is a massive company. You will not be singled out for much praise for great work. If you consistently get above average ratings, you won't get big raises (expect about 1.5%, so less than normal inflation, even raises for promotions are small), you won't have any opportunity to improve your skill set (since you will be in the same role for a few years...it's IBM's way of doing things), and you won't have the opportunity to move within the company (not unique to IBM). Overall, IBM is a place where you are expected to sit back and slowly climb the corporate ladder. Follow instructions and you can make it up at a slow pace. If you have any ambition, go somewhere else.