I've used it.
They say it has way better head tracking than what's come before... but I still felt motion sick after a couple minutes. I recovered after a couple more, and since I was playing a very rough demo from a company that isn't Oculus, it's hard to say how representative that experience was for the device in general. Now, what I can say is that it is quite low resolution compared to what we're used to, though it might not be as bad once the consumer model comes out. I did actually use it with a pair of motion-tracking joysticks so each hand could move around in virtual space. I'm definitely interested in trying it on the omnidirectional treadmill. I have a dev kit in shipping right now, so hopefully I'll have a better sense of it soon.
Given the isolating nature of VR, this will likely never be a huge consumer device. That said, developers are super-excited about it -- at GDC, they had to dedicate a second booth area just so that people could line up to try it. The wait times were E3-esque. And many people who have been in the VR field for decades think this may be just the thing to really deliver on the promise of VR. It's still going to have a tough time gaining traction unless there are some really compelling apps. I think things like Google Glass will have more success... and I guess Kinect One if it really works as well as they claim.