Last night, I downloaded High Fidelity and gave it a try.
Given how resource-intensive the application is, It crashed my older mac laptop. I had to close down a few programs to get it to run. Actually, I gave up on it last night, and I gave it a try this morning. By the time I restarted it, it prompted me to download an update to fix a crashing bug.
Once it was running, it was a vast world with limited interactions. But the wayfinding outlines on the horizon made it feel like I was not completely alone, and I found myself exploring the keyboard controls. I happened upon a few different maps - somebody seems to have loaded map parcels of San Francisco into the world. Another user created a mass slab with a few basic objects on it. Another user created a beautiful and realistic looking sky. Somebody created a basic resort-like structure in a remote part of the world.
I'd really like to try this with an Oculus Rift or a Vive. And, I'm also intrigued by its API - given High Fidelity allows for the programmatic creation of objects in the virtual world.