I've never used the device. I'm just making guestimates based on past experience with head-mounted displays with and without my glasses.
My eyes are about -6. There is about 1 inch in which I can focus clearly without my glasses. Nearer or farther and things get blurry. Most HMDs that I've used are at the outside of this range and are slightly blurry, but barely usable.
They're saying this one has a lens that is closer, but they don't say how far the screen is. I'm guessing the screen will be about the same distance, but the lens will be stretching the screen to wrap around your vision. There's talk about a fish-eye effect that I believe they're using to have more of the pixels in your main vision, and fewer in your periphery, since that's where yours are most sensitive.
Edit: A choice quote I found:
We played without our prescription glasses, so our vision was blurry enough that it didn't affect our gameplay. However, one of our video production members with better vision noticed the low resolution and felt it took away from the experience.
