So, I just built up my third set of the XC-279's. Overall I have to say I'm really impressed. I was looking for a replacement for my previous favorite rim, which was the rim from Shimano's R-500 wheel set. I've re-laced a ton of those into Powertap wheels, and re-done quite a few fronts as well (although with better balls, real grease and re-tensioning the complete front wheels aren't bad). Cheap R-500 sets are drying up though, and it's nice to have some more options besides 20/24.
I liked the Shimano rim because for it's depth it always tested well in various wind tunnel tests, and it's proven to be a very durable rim. It's not particularly WW though, at 460 grams for a 24mm deep rim. Besides the roundish shape, it's also wide for a "standard" rim, 20.8mm. The XC-279 has a very similar profile, not quite as round but close, and it's a little deeper at similar weight. Overall impressions:
-very easy build. I typically bring them to about 70-80% tension w/out even looking at trueness or roundness, and they were amazingly round and true already. Finished tension uniformity was within 5% or so. That's as good as the Shimano rims (which are very straight rims, made by Araya).
-spoke holes were rough. I always chamfer the spoke holes directionally anyway, but if you don't and you're using alloy nipples, you may be in for some problems.
-published ERD seems off. I bought these in a bulk deal from some guy in Taiwan. He listed it as 582.4. I got 582.2. Everyone else publishes 584. This may not seem like a big deal, but with radial lacing and 12mm nipples, you probably want to get this right. I measured 3 rims and they were all around 582.2, +/- .2.
-the two I weighed were right at 480 grams. I don't really care enough about 10 grams to weigh each one. Sorry, WW, I just don't think it matters all that much.
Here's a pic of a Cont Attack mounted. An Attack (22mm) was 23.7mm on this rim, and rim itself measured 23.
I built a set of these for myself as well, 20/24 on Dura Ace 7800/7850 hubs, CX-Rays, DT Alloy front/radial, Sapim brass rear/2x both sides, rider weight 135-140lbs. So far I love the wheels. They should be a great good weather training/bad weather racing/crit wheel. Regarding the "better handling and increased comfort" of wider wheels, though? I really think this is due more to folks decreasing tire pressure and the placebo effect more than anything else. I always ran 80-85 psi on the Kinlin 270's, and I have to say I notice no difference with these rims. Still, I like the rims and would definitely recommend them.