Ive rode Adam Reads bike a fair bit, he runs 200mm Monotrials front and rear and they work really well. They lock very well, and the modulation is the best of any brake out there i think I run a 200mm Monotrial up front and its beast! So yeah double discs do work. I would say the downsides to a rear disc are: Its quite easy to hit a rotor when bailing or catch it while riding. If you hit it fairly bad, then your ride is over, unless you have a spare bedded in rotor (unlikely). Pads are easy to contaminate, like the other night there was few of us riding, and some mong touched Adams disc rotor and it went abit poo for 20/30mins. You need a frame with a disc mount that can cope with a powerfull brake and there isnt many out there Some of the advantages are: Your brake will pretty much work in the wet. You dont have to grind your rim, which will make it last longer. You dont have to faff with Maguras. If you get a disc working well, they are quite low maintenance. Stan.