Been using the Microsoft 4000 for a couple of years now, every time I use it is a joy.
While the general ergonomics is much better than it is in standard keyboards, I've never liked how much force is needed to push a single key. And I don't mean just compared to mechanical keyboards or scissor switches -- the keys needed more force to press even compared to other rubber dome keyboards that I own.
While the palm rest is great as is the curvature, when it came to daily use and pushing the resisting keys, it simply wasn't as much a joy for me as for stickydink.
Edit: I want a minimalist two-piece keyboard, with keys like a macbook..
See: http://ergodox.org/
I was on the Microsoft Natural Elite for a long time. Unfortunately it's been discontinued so I need to find a replacement.