The Advantage (I have the Advantage 2) is simply excellent. It took me about an entire year to fully adapt to it (this is with a very long tail, I was very productive within a few weeks, but 100% adaption took that long). I also type far faster and more accurately on it than on any other keyboard. I think it's the best keyboard available and you'd have to claw it out of my cold dead hands to take it from me.
I tried the Ergodox some others have mentioned in this thread but didn't like the modifier key placements. I've also found that I can type normally (which is reasonably quickly but not at the demonic speeds a few seem capable of) on normal or laptop keyboards, too.
Usually I find myself restricted by the speed of thought rather than speed of keyboard.
