I'm not bothering with retrofitting my existing bikes with discs right now. When I buy a new high-end mountain bike, it will be a different story altogether.

One bike I have will take a lot of retrofitting to get disc brakes on there properly, almost to the point where I might as well save that cash for a new bike. I may have to replace the fork, definitely the front hub and rebuild the wheel, and the back wheel will need an adapter for the frame and perhaps a hub/wheel rebuild as well to get discs operable.

The other bike I bought last month which is my commute bike, I'm not bothering with discs no matter how easy it would be to retrofit, mainly because discs are a thief magnet.