Well technically the ones I have are at least six years old, I think.
The rubber dome membranes should be old and worn out -
but their feel, to me, is quite decent. It's just that a few keys have gone dead on all of them.
Disassembled completely and tried cleaning them several times and got one to work again for half a day but same keys went dead again.
People here (was it you?) claimed they liked the SK-1000REW the best but I cannot stand it - maybe mine is just worn out, I only have one of those.
Working on a Thailand RT7D5JTW right now. It's decent, but to me it's no SK-8000, I can feel the difference even though I cannot explain exactly why. I think it's the initial pressure required to make contact, but the "bounceback" (?) is okay. (Sorry I have to go lookup the correct technical terms for all that). Also I think the keys are much thinner, have thin stems and rattle a little on this compared to the sk-8000 which has nice thick walls on the keys and the stems are like lego-block thick (is that what "sliders" means?)
I am sure y'all laugh at such keyboards but if it's of any help I could disassemble one of each model and show the difference side-by-side in photos someday.