I asked this question of a couple of NASA astronauts, and they estimated M:S was pulling 1.5 G's, maybe two, tops. As others have noted, roller coaster riders experience much higher forces, but for shorter duration.
The "hard candy" advice was based on the idea that low blood sugar contributed to dizziness. In that respect, eating something an hour or so prior to riding would also help. As noted in the Guide, the astronaut I spoke with specifically mentioned avoiding tomatoes and milk, as they're particularly nasty coming back up. (I imagine it's the acid in both that contributes to the unpleasantness.) It also might be a placebo effect, where it's just something to keep your mind off the rest of the ride. Clearly, more experiments in this area are needed. ;-)
Len