I read about (and got sold on) this combo here:
At the time I was considering getting a tubular setup for CX, but decided against at as I'm a broke college student and tubeless would mean being able to race and train on the same wheels, and not pay out the ass for really nice tubular tires.
Anyways, I've been riding Stan's 355 29er rims to Circus Monkey hubs for a couple weeks now. I went straight from running Mud 2's on road clinchers to these. What I noticed immediately:
-I could lower ~10 PSI per tire off the bat. Setting up Mud 2's as clinchers, I'd go 40 front and 45 rear, and generally stuck with that. One race, I pinch flatted the rear. Getting the Mud 2's up and running on tubeless, I go 35-37 rear and 30-32 front (fine adjusting as conditions allow). Edit: At this point, I've raced with my tires as low as 25psi @ 170lb., and ridden around town with them even lower just to test them out. I've bottomed out on tree roots without loss of air on Mud 2's - very confidence-inspiring.
-The wider rim (24mm) means my sidewalls are less likely to collapse onto themselves, even at lower pressure. It's been mostly a non-issue now, actually. You know that squishy, squirmy tire feeling you get when you remount sloppy and high and your tires are on the low side? Used to get it a fair deal on the clinchers, no longer an issue.
-They're light. 410g per rim vs. whatever weight the Aksium rear/Taiwanese generic Deep V was before on my ghetto clincher setup.
-In general I can corner better now and have a bit more control. Also, lower rolling resistance for sure in the grass/dirt/mud with the lower pressure.
-Sealant is the shit. So far had one flat; was riding and got a small cut in the sidewall. Tire lost some pressure but the sealant got it. I was a bit tipsy at the time and riding around tree roots and whatnot covered up on some bike path singletrack, and hit one after the air loss that made the tire burp out the rest. Fair enough - would probably have meant a pinch flat on clincher. Well, taking it home, airing up the wheel twice and turning the wheel so the sealant could contact the area fixed the problem right up. Had I had a pump/CO2 on the trail I probably could have just done it there, but like I said, was drinking beers/riding the bike and no time to consider things like flat tire kits.
-Besides hitting the hidden tree root, no burping issues at all. I've cornered them decently hard on cement during a couple races and they do not afraid.
The initial setup is pretty cut and dry, but it does help to pay attention to detail.
-With a bare rim, you add your Stan's tape (I actually have my front wheel taped up with just electrical tape, after I ran out of Stan's on the rear rim and decided to experiment - no problems thus far but some have reported the sealant may not be friendly to e-tape over the long run). I like to do more layers rather than less - the tape is one of the main things providing a nice air-tight seal for the tire bead, so adding more gives added insurance against the dreaded burp. It's also important to make an X marking on the tape over the valve hole, and then push the stem through while trying to keep a tight seal, because one of the most common ways to get slow leaks is through that hole.
-You add soap suds to the tire and rim (helps to use a scrub brush), and then air the tire with a compressor (or CO2, if you don't have one). No compressor at home? Borrow some air at the LBS or hit the gas station. You only need to do this once to get the tire to seat, so it's not a big deal. You can also use a floor pump if the initial seal is good, but it means you have to do a much better initial job aligning the tire to the inner rim bead channel. Once the tire is getting air, the suds will reveal anywhere where the bead isn't totally seated, and you push down on that area from the top of the tire to help it seat. Go around a couple times doing this: you'll find out pretty quick whether you're using a tire that works well tubeless, and if you've added enough tape.
-If the tire seats (bead pops into place, tire loses a minimal amount of air), then you add in your sealant either by breaking the bead and pouring some in, or getting the sweet little injector gun and injecting it in the valve stem.
-Last step: store your wheels horizontally and flip them over after a few hours. This lets the sealant really soak up into the bead and create a super-nice air tight setup. You'll know your wheels are ready when you're not losing any air, and you can usually spot the areas where the bead isn't perfect because a little sealant will bubble out. If you're leaking out air, the problem is likely one of three things: not enough sealant, not enough tape, or a leak at the valve hole. Once you get the hang of setup, none of these things should be a problem.
A few potential caveats:
-If you swap between multiple wheelsets, the wide rims mean brake adjustment. Solution? Be a baller and just ride wider rims as a rule of thumb. They're better, especially for cross. Major Toms for tubular and A23's for clincher.
-I have the powdercoated non-machined version, and being lazy/not into hard labor, I've put a pretty half ass job into sanding down a brake track. As a result, braking was extremely grabby at first but has gotten better with time. Solution: order machined ones (though not available in white :(), or put in a competent sand job. Otherwise bite the bullet and feel the pain until the mud and grime does the sanding for you.
So, to summarize: doing Sapim Lasers, Circus Monkey Hubs and Stan's 355 rims with the rim tape, sealant and valve, I paid about $420 and built a 1550g wheelset. Not bad. They're lighter than clinchers, and running the same tires between clinchers and tubeless gives off a noticeable performance upgrade in the latter's case. But they're not tubulars: there's no really high TPI baller tires available and some people still have 'burping issues' (which IMO seems to mainly afflict heavier guys and people trying to do tubeless conversions, which seem almost entirely worthless). In the realm of cross wheel choices, though, you really have no clear winner: at one end of the spectrum with clinchers you risk pinch flats but don't have much expense or hassle in setup; at the other extreme, with tubulars, you get the best ride quality and handling but sacrifice in time, money, initial setup and hassle if you get a flat that sealant can't fix; with tubeless you get a nice medium area where setup isn't particularly difficult with access to a compressor (you can ride them about an hour after getting the tire beads seated and letting the sealant do its thing) and pressure can be run at tubular-like levels with less risk of a pinch flat, but you still have to be mindful of not going too low. Being able to afford only one new wheelset, I decided to go tubeless.