Pros
Work environment: IBM is a big family, and your co-workers might end up being real friends, and even family (I've known many marriage cases). It really makes a difference when you have to share so many hours with the same people. Work life balance is great - At IBM, it's important what you do, not when you do it, neither where you do it. Career opportunities are huge, since IBM is so big that you can even change your career path and start over at a total different area, and even a different country. It's really up to you, but there are no limits. You will always work with new technology and innovation is really value. There are awesom programs, such as Corporate Service Corps, which allows you to go for a month to work on volunteer consultant projects at remote places in the world, giving you the opportunity to really make a change. Values, at IBM it's all about values and that is something that matters. Great health packages.
Cons
As in every corporation, you'll have to play a lot of corporate games. That basically implies investing a lot of time on initiatives you know are worth for nothing. And believe me, there will always be a new one. It's absolutely frustrating. Cost, cost, and cost. Everything is about cost reduction and getting budget for anything is really hard. Bonus are a joke, unless you're in sells or any other incentive plan. But for the rest, you can work your hands to the bone, be the top performer employee, and your yearly performance bonus ends up being something like 25% of a monthly salary. Employee benefits tend to zero. Do not expect ever to get not a single free coffee, any merchandising, gym at facilities, ends-year party, or any other type of policies to make you feel part of the company. The strategy is just other.