Well, if you read their site, they claim that the shaft unit won't work on a regular frame.
And, reading Sheldon Brown's
glossary entry on shaft drive, I'm inclined to agree with them:
Originally Posted by Sheldon Brown
A shaft drive requres heavier frame construction around the bevel gears to maintain their precise alignment under load. The drive system is heavier and less efficient than a good chain drive.
For reasons of clearance, the bevel gears of a shaft drive bicycle must be considerably smaller than the typical sprockets used with a chain drive. The smaller size of the gears causes an increase in the stresses on the whole support system for the shaft. This problem is exacerbated by the fact that the stresses from the shaft drive are not well resisted by the triangulated structure of a bicycle frame, because the stresses are perpendicular to the triangle.