Ace
"The highest level Shimano road caliper doesn't disappoint. Made of 1 unit of metal rather than 2 bolted together like the rest of the lineup, I think this helps it avoid the reliability problems that plague their calipers. It has wide pad clearance, weighs about 30g less than the next model down, and has a 3 year warranty. One thing to point out is it's not compatible with 180mm rotors on the rear, no matter what adapter setup you have"