Pluspunten
I have worked for IBM 18 years. I have seen a lot of people go from this company to others in the industry (Microsoft, Oracle, SAP, and others) and the most common thing they miss about IBM is how much material to grow internally your skills is available. Some other people miss the "good vibe", or the ethical principles by what we are guided... (and this last part is no exageration, I have seen directly how top and executive management prefer to lose on a deal rather than winning it unethically. A big differentiator to some other companies in the industry.
Minpunten
IBM is a BIG company. And I mean BIIIG. Decisions, specially those that involve finance can take a lot of time, and work from the people involved. On the same note, career projection is also a long term goal. Planning your career in terms of the next 5 years is not uncommon, and is also a key difference, specially for younger employees.