Pros
I have been able to travel and meet people from all over the world. IBM can look very good on your resume, depending on your job role. IBM is very big on the work at home program, and I have not been into the local IBM office in several years (I usually only end up at an IBM site when I am traveling on business).
Cons
I was acquired by IBM in 2002. I have changed managers on average once per year since then and have only ever met two of my managers while I was working for them (I met two others after they were no longer my manager). I normally have to explain to my new manager what I actually do, and they develop their own perceptions based on what the previous manager told them I do... it's kinda like the telephone game, the 2002 manager describes to the 2003 manager what I do, the 2003 manager describes it to the 2004 manager, etc. and they end up with some vague idea of SOME of what you actually do. In the US, there is very little training given, as they seem to be investing heavily in training the "global" resources. The former CEO made a statement at the beginning of 2012 that US workers are not willing to learn anything new so they have had to turn to global resources. There are many IBMers in the US willing and eager to learn, but there is only so much you can do with free computer based training and little investment in classroom training. When taking vacations or going on extended assignments, it is your responsibility to find someone else to cover your duties and responsibilities (which is hard if you are the only person on your team who does what you do). It is usually easier for me to just stay "on-call" while on vacation, rather than call in favors to get someone to cover my work and then spend a week catching them up to speed on the multiple projects I am juggling.