I'll post what I've posted in other forums I visit frequently. In a nutshell: it's freaking awesome! What surprised me most is how huge are game environments. Watching the Doom 3 corridors, halls and characters at real-life scale is an overwhelming experience. It's incredible to stand in front of an NPC and realizing that he's as tall as you! Then you look down, at the floor, and you see their shoes! And if you look up, at the ceiling, you can see the pipes and lightbulbs... you really have to see it to believe it, it has nothing to do with watching in front of a monitor.
The field of view if huge. It's bigger than a scuba mask (if you have tried one), for me it's big enough to create a believable experience.
Moving your head feels very natural, just like in real life, and it helps to the immersion. And talking about immersion, the 3D effect is way better that any cinema/tv glasses I've ever tried, it's so natural that you forget you are in front of a screen. Each eye receives its own image without any tricks like shuttering or interlacing. It's so convincing that you must see it to believe it.
The first time I tried it I was standing and I felt a bit of vertigo when walking with my Xbox 360 controller, walking slowly helps. After taking a rest I played for almost two hours with no issues.
Bad things? Sure. You can see the pixels and the black spaces between them. When you have been playing for 5 minutes you forget about it, but the resolution is really low and it's difficult to appreciate details at medium and far distances. Also the display is a bit blurry when you look around.
Another small problem is the positional tracking. The Rift tracker can track roll, pitch and yaw, but it can not detect horizontal movements like getting closer or further away from the computer screen peering into something. I mean, if you move your head but don't move the Rift, it will not record it. This will probably be fixed in the consumer version and it's especially important for games where you are inside a cockpit.
In my opinion, this thing is not ready for mass market yet, but I'm sure that with a better screen and true positional tracking, it will change gaming forever.
Should you buy one? It's difficult to answer. It won't replace your current monitor(s) for gaming, but if you have the money and you are the kind of geek that loves new technologies and wants to be surprised, go for it. Anyway if you have the chance, try it. You will never forget your first time. Nothing has ever surprised me so much in my long gaming life.
Take a look at this video, because the reactions of several members of my family have been very similar:
http://www.dbtv.no/?vid=2328907978001