Received mine yesterday. Spent the first hour going through several demos, and at the end of that, I had absolutely no ill effects.
Stuck my dad in it when he came round for lunch, and he spent round about an hour with it, again, with absolutely zero negative effects. The stuff we were looking at though was all joypad controlled stuff, and I'm convinced that really helps to start with, as you can't do instant spin around on the spot movements as you would with a mouse.
Given that my dad is in no way a gamer (his gaming career literally started and finished with Space Invaders and Galaxians), I all but had to drag him off Proton Pulse (a breakout style game), which he really enjoyed.
Had a couple of friends try it as well, both with massively different results. The first, again, had no problems. My other mate felt really bad after literally 2 minutes though later on after trying the other lenses, he felt loads better, so it could just have been that he was somewhere between the and B and C cups, with the B ones offering him the best experience.
My first (and only experience really) of something feeling not right was after about an hour of Half Life 2. Towards the end of that I had to stop, as I was starting to feel a bit dodgy. Later on though with Team Fortress 2 (which I'd have to say is utterly awesome, the sense of 3d in that was something else for me) I didn't have the issues that I had with HL2. Maybe that was to do with the more involved calibration process though - I certainly need to try porting those settings over to HL2.
After chatting about it though, we pretty much agreed that this feels like it did at the start of 3D accelerators, and given a solid consumer launch, it will become a must have peripheral. It really does make that much of a difference to the way you experience a game, and then some. Honestly, watching the YouTube videos of it all conveys nothing of the experience (it simply can't).
The single most impressive thing about it is the sense of scale. Things do look life size. Genuinely. As for the biggest downside at the moment, most people are saying the resolution, and while yes, that really needs sorting (and will be in the consumer release), for me, the screen door effect is actually more jarring (and that may be too strong of a description) than anything else. Being able to see the gaps between the pixels just removes the feeling of utter solidity of what you're seeing.
As for the best demos so far, I'd have to go with (in no particular order):
UnderCurrent
Planet 1
Firma
All 3 give a fantastic sense of scale and of being present in their worlds (UnderCurrent isn't publicly available, but the devs might send you a copy if you ask nicely).
Anyway, feel free to ask if you have any specific questions at all.
