You'd think that Portal would be disorienting and quease-inducing, but I find it plays so smoothly that this has never been an issue, at least for me. Okay, maybe disorienting-- landing out of a portal and trying my damndest to look up from the floor, only to realize I'm actually looking at the ceiling...
Strangely, the First-Person game that's given me the worst (and really only) cases of motion-sickness was one of the more sedate titles-- Morrowind. I don't know if it was my clunky framerate or the FOV or something else entirely, but that game would start me reeling if I played longer than an hour at a time, to the point I had to go lie down to stop the spinning. Go figure?