I had pretty much decided to order an 8kX, as soon as I get a $70 trade-in voucher for my 10m cable extra order (assuming no attempt is made to shirk that debt), but am experiencing some dampers on that resolve, after plugging my 5k+ back in, for a try, after having only used the Index for quite some time.
I had apparently forgotten just how bad the “yellow glare” in the Fresnel lenses is, and regular “content” glare, too. Things also appear just as comparatively “flat” as I do recall, and the distortion is of course there.
…but the biggest shock was discovering just how used I have apparently become to being able to read a line of text naturally, without having to scan back and forth by turning my head like were I spectating a tennis game from a front row seat along the long side.
The small radius of focal sharpness with the 8k/5k lenses was always a problem - especially given the canted design, which greatly increases the need for it to be large on at least the horizontal axis, and I hold it likely it is also largely responsible for the sense of a lack of depth on anything that is more than a pace away, that is inherent to the devices; The lack or sharp edges gives little disambiguity for the brain to work with, and things just become a fuzzy picture hung up a few steps in front of you.
In the index, my view of the room feels correct in front of me - it has proper depth, and everything appears just where it should be, and I am to a greater degree than with any previous HMD I have owned “there” - this with about the same angular resolution as the 5k+, which does not by far offer the same experience. This would, I believe, be a product of little distortion - even a short distance off-centre, in conjunction with the comparatively great edge-to-edge clarity just mentioned.
What I just wrote should not be confused with exaggerated stereo separation, which would stretch depth out, and make things look smaller, losing the geometric verity I so appreciate with the index.
The return from 144Hz to 90 was also vastly more apparent than I would have expected. Given how relatively “meh” I was about gaining the extra temporal resolution in the first place; I thought I wouldn’t care much, but it really made my experience quite uncomfortable, where looking around became a blurry, strobe-y mess, instead of a sense of a decently contiguous optical flow in response to user motion – much like sitting too close to that enormous cinema screen, off of which 24fps content is not only bounced, but triple-exposed. 
(One shouldn’t need to turn one’s head from shoulder to shoulder, to take in the face of an actor in close-up :P)
I guess it’s like going to a new, larger resolution monitor, and then back to the old one – up is not that big a deal, but back down is unthinkable.
…so while I have not experienced an 8kX; After this re-experiencing the 5k+, I firmly think its lenses, which are known to be the same as in the 8k and 5k+, make a very encumbering millstone around the neck of the current and imminent lineup – they will enormously degrade the benefit of the higher resolution for everything other than what is right in a tiny “keyhole” area right in front of you.
I have often said that new lenses would entail a whole new optical path, with different lens-to-screen distance, different size screens, and so on, making drop-in replacement lenses a ludricrous prospect – and that’s before taking calibration into consideration, but if Pimax could make replacement lenses that have the same basic properties, but flatten the field curvature, maybe the situation can be saved…
(Probably still murdering my bank account with that 8kX, rube that I am, but… ho hum… )