Vantaggi
Company with a big name, looks good on your resume. Solid benefits, especially medical. Would help you with immigration issues (e.g., doing a Green Card for you), if it is required.
Svantaggi
Huge bureaucracy. The company tries to squeeze more and more from its employees: most departments that I saw were understaffed and overloaded. The employee population is aging: fewer young people want to work at IBM. Too much management, too few people doing real work. The company has a very complex and burdensome work accomplishment/assessment yearly cycle, so that employees mostly think about how their yearly review will turn out. Hence a lot of time is taken by the efforts to make the review results look good, rather then solving the real problems. IBM T.J.Watson Research Center used to be a great research lab up until 2002-2003, now it more resembles a subsidiary of IBM Software Division, at best, or a bunch of software engineers used to be thrown to plumb holes of other division's lagging projects, at worst.