I suffered from some form of CTS and I can say split keyboards didn't really make much of a difference. It's all about placement of the wrists and keeping a good posture. Also breaks, lots of small breaks throughout the day.

A properly designed split keyboard is aimed at improving the placement of the wrists though.

I found a minor but noticeable benefit many years ago switching to the Kinesis Advantage. I've recently switched to the Truly Ergonomic keyboard; I've yet to decide if it's better or worse overall than the Kinesis.

I think it largely depends on the person. If I rest my hands naturally on a traditional keyboard, my fingers end up somewhere around wdfv okjn. To pull them around to the home row basically involves tilting my hands outward and bringing my wrists inward even further, neither of which is good for either your hands or your typing speed.

If I have pain or numbness (either of which can last for days), just having my hands rotated to use a flat keyboard for a few minutes could be difficult.

Generally, though, if I get pain in my left hand/arm, it's from excessive typing; in my right, excessive back-and-forth to the pointing device (I use a trackball at home if I do any gaming, pad at work, just so it's not the same device all the time). I have never really been able to use a mouse.

Single data point: I had severe RSI problems a few years ago, but I am not completely recovered. For me, the biggest single part of the solution was to start using a break program. I also use a split keyboard and a pen mouse, but the break program is the key. I written up my story here: http://henrikwarne.com/2012/02/18/how-i-beat-rsi/