Yeah, the retrofit is the big deal, though now KH has an external disc hub crank set that is really the true retrofit because you don’t need to change hubs; yes, MountainUni was first.
Disc brakes are good, I have four set ups, two on the 125 mm Oregon Hub and two on the 100 mm Oracle hub.
Kris added some very insightful comments on this thread: Announcing the new KH gear for June 2012
Note that he suggests getting a more powerful brake vs getting a bigger rotor.
Also, he suggests that Maguras would be a better choice for unis that get thrown down a lot in super tech DH scenarios. This comment gives me mixed feelings as I have broken a Magura hydraulic slave fitting from a rock hit and two levers from rock hits, so lever wise it’s a push in terms of durability, and it seems to me that having a brake exposed on two sides is more risky than having one exposed on one side…
One comment that I’d make is that having crank mounted disc brake is not only easier to retrofit, it is also easier to remove/adjust, and you will have a stronger wheel. Not that I have had any wheel failures with the Oracle hubs, but the disc brake side spokes are very steep.