I do not get dizzy, but there is some mild awareness when having played the Rift.
It can be compared when I ride a roller coaster in a teampark, I'm not sick nor dizzy afterwards but there is a mild sensation of disorientation and very mild nausea depending how severe the ride is.
At least that is what I get riding any roller coaster in a theme park.
I do not feel anything sitting in a room or doing static things with the Rift, but I guess when doing adventures (flying a Mig with crazy turns, space ships, massive rollercoasters,...) the fact that you get a little sick or disorientated is a testimony how 'true' the virtual reality experience is, because in real life doing those things I would get the same feeling, and that varies by person.
On a side note:
The reason people are getting sick doing these things is simple; the organ behind your ear that registers your physical balance.
When you eyes register conflicting things over what this organ registers, your brain is in conflict and hence turning sick (reading in car, backwards on a train, etc).
How sensitive your brain and that organ, I can't recall the name, varies strongly by person, some are VERY sensitive to it and some are not. Medicine against car sickness or travel sickness tones down that organ or brain effect (not that I'm implying taking this to play the rift, that euh questionable)
** Looking at a static object while moving (or believed to be moving using the rift) is one thing, but unstable frame rate or latency causing a stop in imagery accelerates the above discussed aspect so much it's almost a guaranteed trip to the toilet and vomit session, pardon my directness.