I have a pair of the new Ultegra Di2/Hydraulic levers on my Crux right now and they are nothing short of amazing. I had the SRAM Red Hydraulic discs for CX season, up until the recall and was impressed with them at the time.
Big advantages that I like over the SRAM brakes are how snappy the levers return, they seem to have a heavier spring to return them to their starting point and this give great lever feel.
I have had some brake howl when the rotors get wet. It doesn't effect the performance or power, but it's not ideal. It goes away quickly.
Di2 is what is. Love it or hate it. This is my first Di2 set-up and so far I have been happy with it, but not necessarily blown away.
I think the Shimano set-up is a hair heavier than the SRAM Red hydros were.
I guess the big question is, what's in store for the upcoming SRAM hydraulic levers and calipers.
I'm in the "I love Di2" camp. I've re-programmed the levers to swap buttons too.
That's why I want Shimano hydros.
How about switching wheels - do you have to shim hubs?
I do NOT have Shimano hubs currently, and don't plan on it.
I have some Roval CLX 40 clinchers that I have been using, and they use a center lock hub, and there fore Shimano's rotors.
For CX when I was using the SRAM brakes, even with the SAME wheels and the SAME rotors, I found that I still had to make minimal adjustments to the calipers each time. In my opinion, it's a far easier and faster adjustment to make than a canti or other rim brake, so I take that in stride.
That has been my experience on MTBs as well, it's never perfect wheel to wheel.
I do not have much time on the clincher 40s yet. I rode CLX 60s all last year on the road and loved them. Then had a 2 sets of the CLX 40s tubular for CX, but they didn't see a TON of use, just because of tire selection. I'm a big fan of the center lock hubs, and since it's a DT Swiss freehub, it's easily upgradeable/changeable, etc.
I was pretty hard on the wheels and raced that at CX Nats when the ground was frozen solid and full of hard edges. They bottomed out regularly each lap, but I'm happy to report no damage.
They felt stiffer and faster than my Velocity Major Tom Pros or Comps, which isn't surprising, but worth noting.
I am running SRAM bar end shifters on my RLT and currently have mechanicals, but the plan is to go to some TRP Hylex in the future. My brother put some Hylex brakes on his Fargo and so far loves them. I did not want the cost of switching everything to SRAM 11 or Di2 so using barend shifters is by far the simplest way to accomplish that and leave my braking choices pretty wide open.