I might be the minority here but I like to use the stock Ultegra pads when riding my aluminum wheels (assuming yours are aluminum?). I've tried other brands with them but nothing gives me that bite like the Shimano pads that came with my Ultegra 6700 brakes. Like redfooj said above, be sure the rims are not slimy and give them a once over with some acetone and a scouring sponge to clean the braking surface of your rims and the surface of the rims. I'm a fan of disc brakes but feel rim brakes usually give you more of a bite as opposed to a "squishy" feeling.