The Kinesis Advantage works fairly well for me on the Mac. You can change key layouts on the keyboard itself (as opposed to the OS) if something isn't to your liking.
There is one thing that I missed though: play, pause, ff, volume and brightness controls on the keyboard. I haven't worked out a solution for volume and brightness, but play, pause, and ff I do with keyboard shortcuts through Quicksilver.
Just wanted to clarify: it looks like you _can_ set different key bindings for screen brightness in the Mac's preferences. I can't find similar for volume controls though...
I used to be, and the Kinesis works great there. It is a super configurable keyboard. You can swap the definition of any two keys (make enter -> backspace, caps lock -> escape, etc.)
The only OS that I've had issues on is Windows. There's a USB3 driver issue there. But I don't program in Windows, so doesn't affect me.
Also agree that most people here would benefit from a strength training routine.