Pros
Because IBM is a such big companies, there are many other career opportunities within the company once you have a foot in the door. You can spend your entire career at IBM holding tens of positions. IBM also has pretty good benefits such as 15 days vacation days, 6 floating holdings in additional to federal holidays. Insurance plans are also better than many other big companies. If you are a slacker and want to be lost in a big ocean, IBM is also a place for you. I knew somebody who were on the bench for 6 months doing nothing and still had their job.
Cons
It's hard to be recognized for your achievements because it's so big and your manager has many too many direct reports. Many of your peers are incompetent professionals. If you are lucky, you will be staffed a project surrounded by great people whom you can learn from. However, most of time, especially in Public Sector, you will be staffed as a body on project that was set up to failed surrounded by slackers. Work life balance was very for me while working at IBM. For Christmas in a row, I had to work for releases schedule to go live in the week of January. I was in GBS SCM Public Sector.