Having benefitted from these forums as well, here are my two cents on the assessment centre, for those preparing for the ordeal…
First of all, know lots about IBM – its practice areas, its competitors, culture, values, key personnel, history, advertising, etc. I was very lucky that my interviewer didn’t ask me a lot about these things, but I know other applicants were. I could easily have failed the AC if I’d gotten one of these other interviewers. Also, interviewers have very different styles… some very easygoing, some more hostile (so if the interviewer is firing tons of tough questions at you, it doesn’t necessarily mean the interview is going badly).
Second, if you haven’t figured this out by now, teamwork is the main thing they’re looking for. Are you competing against your fellow applicants? Yes and no; theoretically, they say they could take everyone or no one from the assessment centre, depending on the quality. But in reality they’re not going to take everyone. The ACs are a stressful, somewhat inhumane, somewhat farcical enterprise, but (as qwerty mentioned above) the best (and perhaps only) way to get through it is to be nice to everyone, find ways to help get the best out of everyone else while also making your own contributions. There’s no way to sabotage your fellow applicants without looking like an a$$hole yourself, so just be positive and genuinely try to solve the tasks you’re given as a team.
I should also note that there is some randomness about the selection, it seems. There were people who didn’t make it through the first cut in my AC who seemed really smart. The assessors probably do a good job, but far from a perfect job of making selections, so try to take it in stride if you don’t make it. There’s always something else out there, always an organization that will recognize your talents.