Big game changers usually don't do to well until companies ease into the change. The Rift is going to straight up offer a new way to play at inevitable large prices. But it may do decent in the current market. I can see 500,000 being sold maybe on day one with around 2 million by the end of its first year.
With that said, my feelings for the devices success end there. At the most it will probably carve a niche market. Same with the new Onlive, we aren't ready for that stuff yet, and they will only have early success because of hype alone and then die off from the looks of it. Same with gaming phones, the first online, ouya, Virtual light guns, etc.