I love having a split keyboard. It feels far more ergonomic than a single piece keyboard. Just relax your arms and put each half where your hands are. No having to bring your hands together and angle your wrists to get your hands in position.
The other piece of the puzzle for me is curvature. On a flat keyboard there is more stretching or physically moving but adding in a bit of curvature helps cut down on that.
For a few years now I have been using a hand wired Dactyl-ManuForm [1] and it has been great. At least for writing / programming. I go back to a regular keyboard for gaming purposes.