I've got a pair of nitto bullhorns, and I recently picked up some used SRAM bar end brakes with internal cable routing. Like these:
http://www.amazon.com/TT900-Bicycle-.../dp/B0012MVMSY

I have read mixed things about drilling handlebars for cable routing. It seems like people recommend against it on principle, but I haven't read of anybody having a problem with it in principle.

Has anybody done this successfully? Anybody had any problems with it?