IBM in the US is shrinking, and as an employee you will compete with your coworkers for survival. Many people have been at IBM for 30+ years. They have survived many layoffs. If you want to keep your job, you need to be better then they are in the eyes of your manager. Being talented and hard working is not enough. You need to sell your ideas to managers. Most managers were talented engineers at one point, but managers are shuffled often, so most manage groups whose work they have a limited understanding of. If you can't explain to your non-technical relative why you are valuable to IBM, you will struggle to keep your job, no matter how valuable you really are.
There is a constant flow of people who join IBM right out of college and are gone within four years. About half find better jobs because they have IBM on their resume. The other half change fields entirely, or go back to school.
A manager being laid off is unheard of. Non-managers generally distrust what managers say about layoffs, because managers are invulnerable and so have no empathy. The managers I had took every opportunity to say that yearly ratings were based on your performance and not compared to your pears. In other words, everyone under a manager could get a high rating. This was to fight a persistent rumor that managers can not give high rating to everyone. I believe the rumors are mostly true: A manager could give everyone a great rating, but other managers would question that manager's judgment, so in practice no manager does it.
One trap to watch out for is moving to a town like Poughkeepsie NY, where IBM is a large chunk of the local economy, when IBM is at a hiring peak. The housing market moves with IBM's hiring or layoffs. If you buy a house when IBM is hiring, and are forced to sell it a few years later when IBM fires many people, the loss you will take on the house could easily be greater then a year's salary. Ouch.
IBM tends to use technologies that are internal or not the industry standard. Often the IBM way was invented before the industry standard way, and often the IBM way is (or was) better by a small margin. Keep in mind that IBM may not be the last place you work before you retire, and make sure that you learn some skills that will allow you to get a job elsewhere. If you spend ten years at IBM and become an expert in the IBM method of making chips, you may find that you can not pass an interview to get a job making chips somewhere else.
