Pros
For a large company with over 330,000 employees, IBM does many things right as an employer. They pay competitively and have a solid benefits package. They have a great internal training program for things like management and project management. They delegate much of the human resources (HR) responsibilities to the first-line managers, so that key HR decisions are being made by those with the most knowledge about the employee instead of by HR representatives who couldn't pick the employee out of a line-up. They have a strong mentoring program (people who say they don't have a strong mentoring program probably weren't willing to put in the effort and time necessary to make those mentoring relationships fruitful). IBM is large enough that employees can have multiple careers without ever changing companies or having to rollover their 401k.
Cons
Anytime there is a large number of people, communications become harder. Project managers or engineers might think about the number of communication channels, which is n(n-1)/2 where n = number of stakeholders (Ref: PMBOK, 3rd ed.). For 330,000 employees, that means IBM has 54.4 billion communication channels. Obviously that is difficult to manage and work within. Recognizing that there is an inherent challenge in the liquidity of knowledge and information can help employees find ways to solve those problems, such as developing wikis for small teams. The other downside to having that many employees means that every time the company spends $3 on each employee, you just spent $1 million. Many employees don't understand this problem, and instead complain about only receiving free ice cream in the cafeteria after project milestone is reached when their cousin at the unprofitable, but well funded, start-up is getting free coffee and soda every day.
