Pros
A very large company with many career potentials. Worldwide presence makes it a great company to work for if you have an interest to work overseas. IBM was ahead of its time in so many ways including embracing social, open source, and cloud as the core of its business. Company has very smart people working in it. Work/life balance is good.
Cons
As much as the company was ahead of its time in many areas: cloud, social, mobile, and open source, it didn't do enough to make a lasting impact in the industry. There are smart, strategic IBMers who had long identified those up-trends. But senior management hung on to status-quo, focusing on mainframes, middle-ware, and once high margin hardware business. There is little to no career development. Most people are "tenured" are staying to retire. Executives are compensated based on earnings per share. This inherently steer them to cost cut in places where it matters most: high quality hires, education, and technology investment. Short-term gains and complacency lead to long-term decline of the company. Most executives are just waiting for retirement, leaving a shadow of a company that IBM once was.