Vantaggi
Frequently a leader in technology development, though one's exposure and participation in that effort depends upon which part of the company you work for. Most of the management is "sane", the real wackos are filtered out (usually). The company has been around so long that they've seen everything and don't often panic when situations (of all kinds) occur.
Svantaggi
So large that the goals of different parts of the company often conflict. Benefits, vacation, pension, stock purchase plan, medical, etc. have all been squeezed in recent years so the fringe benefits of working for IBM are not so great. They're also working hard to transfer jobs out of the USA to China and India, this makes one's future with the company in the USA less stable. The level of expertise of the technical talent can vary widely, many parts of the company are not attractive to really talented people who can get jobs elsewhere.