The Dura Ace hydraulic disc brake are as advertised. They have built on past success and implemented a near perfect solution for the road bike genre. The 9150 calipers have flat mount form which reduces weight and allows for a more inconspicuous fit to the brake mount. Braking is smooth with plenty of modulation. The rear disc rotor is 140mm and the option for a 140mm or 160mm exists for the front brake. For increased stopping power it is advised to go with the larger rotor on the front. This tends to balance well with the 140mm rear rotor.
The disc brake rotors are designed with extensive cooling fins radiating inwards for maximum heat dissipation when descending those steep mountain roads. Shimano has always used mineral oil with their braking systems going back to the original mountain bike offerings in the early years. Mineral tends to reach a bit higher temperature than an equivalent DOT fluid system, however it is much easier and cleaner to work with as it is not caustic and will not damage your frame of components in the event of a leak. Shimano compensates well for the added heat buildup with brake pad fins along with the rotor cooling fins.
Safety has been built into the disc rotors to reduce the chance of accidental injury. The cooling fins and rotors have sufficiently round edges to reduce cutting injuries. In addition, the inward radiating cooling fin profile fills in the rotor to an extent where it would be unlikely to get objects or fingers caught in the rotor. With cooling being optimized, heat buildup will be lessened with the Freeza sandwich material technology. Cooler rotors mean less chance of injury in case of accidental contact.