I'm still waiting for my HDK2 to arrive. I did try it at E3, though the HDK2 demo goggles they had at E3 still used diffusion film technology to reduce the screen door effect while the commercial version of the HDK2 will ship with image quality enhancements to reduce the screen door effect that is supposed to be more effective than diffusion film technology.
When I tried the HDK2, image quality wise it was really no worse or better than either the Rift or the Vive, though the screen door effect was more noticeable on the HDK2 than on the Rift or the Vive. Since the HDK2 is open source and compatible with OpenVR/SteamVR, it will work with any game the Vive supports and since Rift unlocked their software to allow Vive compatibility, the HDK2 should also work with any Rift game.
Another caveat since its open source is that there will be more choices for 3rd party hardware.
One last thing that does put the HDK2 above the Rift and Vive is upgrade ability. With the Rift and Vive, when a new version or update comes out, you will have to buy new goggles. Not so with the HDK2. When an upgrade comes out, (like Leap Motion) all you have to do is purchase the upgrade kit and upgrade your HDK2 goggles to the new version without having to buy a new set of goggles.
The only thing I wish all three VR goggles had was the ergonomic comfort of the Sony Playstation VR goggles. Those suckers you can wear for hours and not feel discomfort.
