Well, Eureobike is here and almost gone and SRAM's highly anticipated hydraulic shift/brake lever didn't make it to the party. No word on when it's going to get here. What does this have to do with our plans for CX and discs? Plenty: until there is a readily available and economical and good performing hydraulic disc option, there's no great reason to offer a disc bike. You must keep in mind here that my perspective is that a cross bike is a bike designed for best performance on a cx course, and training for cx racing. Other uses are incidental and subservient to cross racing use.
For a short time still, I'll have the chance to stand on my soapbox and wag my finger and say "I've got more experience racing cross with disc brakes than most people, and you kids get off of my lawn, and stuff!" Last year when I wrote a midseason blog about it, it was mostly about what a logistical pain they were proving to be. This is still true. It's not the entire story, and if there were some completely rampant advantage that discs had, I'd sacrifice the convenience for that advantage.
Mechanical discs work pretty well, but not very well. They have pretty poor pad clearance, meaning the pad is never more than a hair (if that) away from the rotor. If your rotor/caliper interface isn't aligned PERFECTLY, your pads rub your rotors. It probably costs a few watts, but more than that it's annoying. Of course on nice mechanicals like the BB7s that I used, you can easily dial out pad contact, but this comes at the expense of power. Power goes away QUICKLY as you dial your pads out.
In soupy mud, they don't stop. They just don't stop. I tried both the organic pads and the sintered pads and neither stopped in mud. The organic pads wore out crazy quickly - in one muddy race (Kinder Kross), the disc didn't even pretend like it was going to stop me by the time the race was over. That was fun. The sintered pads wear out less quickly but still quickly - enough to change braking dynamics during the course of a race. Hydraulics self-adjust for pad wear.
Rotors go out of true really easily. You have to be super careful with them at all times, as the slightest warp/bend is going to cause them to rub the rotors like mad. They also squeal like a stuck pig in almost all conditions. I can't tell you why the hydros on my mountain bike are quiet but the discs on my cx bike were the opposite of quiet, but that's what it is. Maybe it's pad pressure or I don't know. Pads are the same. LOUD. Properly toed-in cantis will neither shudder or squeal.
Cantis are lighter, and the wheels are lighter, and the frames and forks for them are lighter. Last year my bike was around 17.5 pounds - not much different than the disc SuperX that Tim Johnson used. This year's setup is is a bit over a pound lighter. I'm not a weight weenie in any respect, but a pound's a pound. Since the logistics of being able to use RFSW38s (tubulars) as both road and cross wheels means that I WILL do so, I will have a somewhat significant weight loss beyond that.
Discs were of course supposed to collect a whole lot less gunk than rim brakes (I should stop saying "cantis" because mini-v's are a great alternative too). It may be the case, but judging from my bike compared to others last year, this wasn't the case. I also watched a lot of races to see how often Tim Johnson, using discs, pitted relative to others. He pitted no less frequently than anyone else. Maybe it wasn't the brakes that caused him to need to pit, but the discs didn't save him from having to pit.
We're not against discs at all per se, it's that in our minds they aren't yet the best solution. I just read a review of the new Dura Ace, at the end of which the reviewer said "with gruppos like this and the new Red, I'm at a loss to figure out what exactly electronic shifting does for you - these mechanical groups shift angelically, and electronic weighs more, adds some complexity, and costs a TON more." I never used canti brakes other than Euro-X (and not very much at that) and Shorty Ultimates. I know there have been some super crap cantis that people have had to use for a while, but comparing Shorty 4s to Shorty Ultimates is like, well, even I don't have a metaphor for that. Shorty 4s are tough to set up, a pain to maintain and don't work well, while Shory Ultimates are easy to set up, easy to maintain, and work crazy well.
I'm sure in 5 years we'll all use discs, because that's the way these things happen, but we don't think it's the best solution for cross yet.