I know the HHKB isn't technically a Mechanical Keyboard but that being my latest excursion, it's been rather interesting to use and work with.
I tried everything and still have quite the keyboard collection (Cherry MX-5000, industrial Model M, ****load of Model Ms, some ALPS based prototype, ...) and after years of working with Model Ms and not liking other keyboards, I discovered the HHKB's Topre switch and never went back. It's funny because Model Ms and Topre are completely different: it took that to keep me out of my beloved Ms. I've got three HHKB: one Pro 2 (which I don't use anymore) and two Pro JP. I use the japanese for the shorter spacebars, which allows for easier reach with the thumbs of the modifiers (and there are a lot of modifiers on japanese keyboards, so although I use it in QWERTY mode I get to have HYPER and SUPER modifiers, for example).
I'd switch for a split HHKB Pro JP (if it had the exact same layout, but just split) but that's basically it. I found my "endgame" a few years ago and must admit I kinda stopped obsessing over keyboards as much as before. I still have all my keyboards, some fancy keycaps etc. but I hang out less here.
Still checking in once in a while to say hi to newcomers and old chaps as well : )