I will post my experiences here, just in case they can save anyone a bit of trouble.
I had one of the first DK2s to arrive in the UK, and must have been one of the first players to experience ED in VR. I skipped CV1 because several DK2 owners were disappointed with the upgrade and complained about the god-rays. But when the Rift S came out, I decided to give it a go.
The improvement in clarity was huge, but there were also huge disadvantages. The horizontal FOV was so decreased after the DK2 that I constantly felt I was peering at a distant world through a telescope, and the device was also (to my great surprise) less comfortable. The other strange thing was that the Rift S made me sick, which the DK2 had never done (in Elite, at least). Then there were the god-rays, which I found really distracting, particularly in Elite. Because the cockpit is illuminated and the exterior generally dark, you get constant smear rising up at you from below, and there's nothing you can do about it. Very, very irritating.
At first, I was willing to put up with all this for the sake of the extra clarity, which really did blow me away. But then one day I was looking around and admiring a planetary surface when I realised to my horror that the interior geometry of my SRV was warping as I moved my head. Adjusting the position of the headset lessened the problem slightly, but I couldn't make it go away completely, and once I'd noticed it, it completely destroyed the reality of the experience.
Nobody seems to talk about this, but for me solid geometry is essential for a sense of presence. The Rift S felt less like being transported to a world than looking at one through a distorting fish-eye lens. Then there was the nausea, which only seemed to get worse, along with increasing eye-strain, which I had never experienced with the DK2. Eventually I realised that this was because my IPD is about 3mm out (at 67mm). The DK2 has no IPD adjustment, so I thought I'd be fine, but the Rift S lenses are apparently less forgiving in this regard - so be warned if your IPD is out of range.
What with one thing and another, the Rift S just killed Elite for me. Although I could see more detail, the sense of reality and wonder were somehow gone - and, after all, if it was just detail I wanted, I'd play on a monitor.
So I sold the S and tried an Odyssey+. This had better blacks and an IPD slider, so no more sickness. However, it felt cheap and uncomfortable, and the fish-eye distortion was even worse than in the Rift S, which made it a complete non-starter for me. So I soon sold it and ordered myself an Index. I just got the headset and a single base-station for Elite and a few other sims.
The first thing that strikes you about the Index is the build quality. After the S, it's like moving from a Kodak to a high-end SLR. The geometry is pretty much rock-solid, with just a trace of distortion if you really look for it, but not nearly enough to be immersion-breaking. The FOV is far better. There are a few god-rays, but I find them far, far less distracting than they were in the Rift S. In the Index, I hardly notice them at all, whereas in the S they bothered me pretty much all the time. The sound, of course, blows the Rift S away (though the S was amazingly directional - still not sure how they managed that). It's not just that the Index has more power and better bass, but that it separates everything so much more clearly. So in Eurotruck, for the first time, I can play loud music and still hear my indicators ticking above the rumble of the engine. Superb.
So for me - given that my priorities were IPD adjustment and solid geometry - the Index was worth its high asking price. I don't know if it's the FOV, the lenses or the sound quality, but it's the first headset since the DK2 to really make me feel I am in a different world, rather than just looking at one on a fancy 3D screen. As for the blacks, although the Odyssey+ was impressive in this regard, I found I adjusted instantly and now think the blacks in the Index absolutely fine.
Of course, all these things are highly, highly subjective. If all this has taught me anything, it's that the only way to judge an HMD is to try it for yourself and see if it 'works' for you.
Which, I'm afraid to say, makes this long post rather a waste of time...
