Yes, I know how they work, my question was more geared towards what people's feedback was in terms of quality and how happy they are with them compared to other brands.
I'll shed some light on this. Importantly (and you may well know this and apologies if you do) you can currently only use the Zeis, Freefly VR or other phone based VR sets with litchi. Viewing the phone without the VR mode through the googles just doesn't work. DJI Go currently has no method to view VR (or at least on an iPhone... maybe on Android via 3rd party app?).
The experience when using the Litchi VR mode with Freefly VR Beyond is excellent. The screen is clear, bright and very little blurring at all. You also get a slight 3D feel as the overlay is in 3D (the actual video of course is not). You also get to use Litchis head tracker mode which means, when set, you look left and the phantom yaws left, you look down the camera tilts down (you get the jist and again sorry if you know this already). All this is just using the phones accelerometer so no extras needed.
Things to note - the pixels of course are enlarged view the lenses. Most smartphones capable of running Litchi/DJI Go have no issue with this as they all have very high DPI screens so have tiny tiny pixels but older phones will show more pixels of course and reduce quality.
Light from the outside world is cut out really well (depending on the VR set). This is really handy on bright days when even a sun shade still has reflection issues.
Bad points of phone VR is that you don't have full control of the app easily. No way to change camera settings or touch the screen easily without removing the phone. For me that is the worst part. HDMI goggles don't suffer from this as the phone/tablet is still fitted and active so you just take the goggles off and use the screen. HDMI goggles will feel a bit more natural in terms of view also as there is no split line like the phone VR (you have one screen per eye). This means a little less blur.
HDMI goggles are also great for sharing your experience with others.... you fly goggles and they see your screen or vice versa.
Personally I would advise buying either a $15 - $40 phone VR set first or even just google cardboard to see how you find the litchi VR experience. If you like it then buy a better set like the Freefly VR or Zeiss if you feel inclined (they are very over priced imo tho).
If money is no option though just buy HDMI goggles like the latest Fatshark Doms or equivalent (I tried Vuzix iWear at the Drone show and was impressed with their clarity... pricey and bulky though).
If you have never tried goggles of any kind at all I would highly recommend going to a shop that sells them so you can try them on and see how you feel wearing them.