Pros
On the plus side, IBM does offer a good overall package to its employees and there are certain benefits to working for a global name; it can add something to your CV, provided you have the good sense not to stay there too long!
Cons
As has been stated above, a lot of work at IBM involves talking (and reporting) about what you're going to do as opposed to actually doing it. Regretfully this leads to people being judged more on their personality than their actual work. I worked for Sun Microsystems prior to working for IBM (before Sun shut down our UK hardware R&D offices) - if ever there were two more contrasting companies to work for, I'd be hard pushed to find them. Sun relished individualism, nurtured creativity, and rewarded hard work, whereas IBM was more concerned with squeezing you into a box and talking the talk as opposed to walking the walk.