https://www.flickr.com/photos/winkyintheuk/albums/72157665522087839
I've now got a month-or-so and over 1000km on my new Dura-ace disc-braked all-road bike (see link above). Here's my experience:
1) They work great in the dry. A bit better than my SR on Exalith-treated rims, but not much in it from a practical perspective, really.
2) They work OK in the wet, but once saturated, the initial bite is much worse than when they are dry. In the wet, once the water is cleared, they are better than my SR rim brakes, but I actually expected less performance drop-off in the wet from discs than what I am experiencing. This has been quite disappointing as wet-weather performance was what I was really after.
3) Clearances appear to be too tight in the calipers. Small bits of debris from wet roads seem to very commonly get lodged somewhere in the calipers, causing a scraping and pinging noise from the rotor. It's not the pads rubbing, as it never happens on dry roads, and is not related to braking. If I ride through a puddle, the noise is almost certain to start, and lasts for a few hundred metres before whatever was stuck eventually clears itself. Next puddle, it starts all over.
4) The deafening squealing of the pads in the wet is absolutely infuriating. But for the performance of the brakes, this would perhaps be a deal breaker for me. I've tried both sintered metal and resin pads. Hard braking, light braking, everything in between. No difference. They squeal like banshees until they are well and truly hot and dried out (which only happens on the most severe downhill stops).
5) Some have advised that there is a mystical "bedding in" process that must be followed. I have had much conflicting advice on this, but don't really see how it is much different to just using the brakes. I also have absolutely no time for this level of stuffing about. Kind like the reason I won't use tubeless road tyres.
6) I replaced the front OEM resin pads after about 600km. They were more than 1/2 worn. I expect longer life from the new sintered pads. The sintered pads also seem to have better "bite", but both are compromised in the wet.
7) The return springs on the pistons and pads don't really seem strong enough. It sometimes takes a few seconds for the pads to clear the rotor after I release the brakes. Yes, my pistons might be dirty (after just 1000km!), but really, who needs this issue? I'd gladly put up with some more lever force for a more positive return.
All in all, I'm glad I went to discs for my all-road/winter bike due to better (but worse than expected) wet-weather performance and the fact I'm not wearing the rims, but on the basis of my experience, I would never contemplate discs for a "summer" carbon race bike that was going to see mainly fairweather riding. I just don't see that the upside is worth the compromises.