MP 1 gave me some very bad motion sickness, as did MP 2. Other games to do it were Duke Nukem, Half Life 2, Timesplitters 2, and certain stages in Monkey ball 2. Quake and Doom never did it to me, Geist didn't either.I think much of it has to do with a very fast framerate and the way the viewpoint moves ("viewbob", if you know what I mean) when walking around.