A developer from work bought one and I've tried two demos, one was a rollercoaster and the other was a Unity demo that had a empty house in a mediterranean island. It's really immersive. When you see someone else using it, it seems like they are faking it or are really high, but when you put the thing on your head, it really fools your brain, you have a very real sensation that you are there. I have fear of heights and the rollercoster one made sweat, I almost felt the G-force... my friends laughed watching me suffer. It's something way beyond common 3D glasses.
But like others said, the resolution is low, you need to setup the glasses for your vision angle and body height and is very boring to do so and you have a very strange feeling of disconnection with your body. The rollercoaster one had you "legs" when you looked down, but it was very bizarre because your brain somewhat believes that's your legs, but they don't respond and you feel like you're suffering some kind of paralysis... or like you're inside John Malkovich. The motion sickness is really a issue... I felt really nauseated and a friend that's prone to motion sickness almost threw up after 10 minutes. The eye strain is very hard too, since your eyes are glued to a LCD screen.
Another funny effect is that you perceive much more the polygonal nature of 3D graphics. The Unity demo wasn't that low poly, but because you have such a amazing depth perception, you feel like you're inside a cardboard world, specially when looking at organic things, like plants.
I think it's hard that will be a success at consumer level. It's amazing, but it's still very cubersome as a product. It's kinda like a light gun: it's cool sometimes at the arcade, but it's tiresome to have it at home.