A friend of mine was an early Kickstarter backer and brought his Rift around to my place try out with a few friends. We tried out Half-Life 2 and as soon as I put the Visor + headphones on I was blown away. My brain's "location neurons" were 100% fooled into thinking I was in the HL2 world. Within a few seconds I had lost the "background" feeling of being in a small room, and accepted I was in a large, open-aired train station (HL2 intro). Unbelievable.
The early Oculus DevKit certainly has some issues: no translational tracking gives me strong motion sickness, high persistence display makes the scene blur as I move my head, low resolution produces screen door effect. Regardless of these downsides, after 10-15 seconds of use I accepted that VR will be the "next big thing."