It's not mechanical, but the Microsoft Natural Ergonomic 4000 is worth trying because it sits comfortably in the lap. For me, this makes it better than any keyboard with a rectangular layout. No wrist arch. No "where do my elbows go?" In EMACS the symmetrical Alt and Ctrl keys are a huge advantage.

Like many Microsoft products, this is something that comes out of the fact that employees spend a lot of time typing at the computer. It's the kind of product that comes from dogfooding.

It's the AK-47 of keyboards.