Thanks! The u-brakes are surprisingly great. I hadn't heard of them either, I just looked around BMX forums for a modern u-brake with large clearances and these kept popping up. I ended up getting their levers too because the NOS Ritchey's I originally had in mind felt a little too vintage.
I have serious envy levels on that Karakoram, mate. It's the perfect neo-retro MTB. How are those Odyssey U-brakes though? Never heard of them before.