Pros
Big company, slow work environment, casual and laidback. Decent compensation, and they generally have good benefits. You can access education, if you need to, and they might support your career development if you're one of the favored employees.
Cons
Lots. The company is squeezing the lifeforce out of their US employees by outsourcing whatever they can overseas. Some organizations in IBM still maintain decent work life balance, and it also depends on your role within the company, but that balance is going away with added pressure to be more productive with less headcount. Before my time there was over, I was working 60-80 hrs. a week. The management in my department was clueless and had no idea how to load balance the workload or skill sets. They just absolutely SUCKED! And most of the senior engineers there just sucked. They couldn't figure out how to fix a bug if it costed them their lives. It made the workload even more unbearable since you had to make up for the slack in skills. Also, IBM's upper management has been cutting their U.S. expenses to the bone. There is never any money to do anything there. This has been going on since 2000, and I suspect it will get worse with time. With less money, expect less employees and more work.