Based on my Strava data, I put about 2,100 miles on these tires in 2012 and 2013 before finally changing them over two weeks ago before the first mtb race in March, 2013:
Let's just say I wasn't nice to these tires. I put them through a lot of abuse, bouncing through rocks without picking the best lines, riding in deep snow and ice, mud, sand, and multiple 100+ mile rides. In all that time, I had one flat when a 5 inch spike went through the rear tire. It was easily fixed with a Genuine Innovations tire plug resulting in almost no air loss and I just kept riding the tire for another 5 months.
The sidewalls on these German-made tires show a what seems to be a lot of wear. I noticed that after just a few rides, the sidewalls close to the tread looked "worn", with the black casing looking like it was getting chafed by rolling over rocks, etc. Despite that look, it didn't seem to affect any performance and didn't decrease the tire's resistance to sidewall cuts (I didn't get any). I didn't get any sealant or air leak.
In fact, this is just about exactly how the tires looked on the sidewalls after the first few rides:
|Some sidewall scuffing seems normal for these tires - it didn't affect performance or ability to hold air.|
I never had any problems with these tires holding air. After a few weeks, they might lose a few pounds of air, but that was pretty minimal. I found myself running lower and lower pressure during the winter time. When I finally checked, I was below 20psi in both the front and rear tires. The low pressure was great for traction and ride comfort on my Stan's Crest wheelset. After running them that low, I started to keep them low for use on my Scale hardtail. I increased the pressure a little bit (to 24psi) on my Spark full suspension bike.
These tires's added sidewall protection does make for a "stiff" feeling ride if I ran them at a pressure I normally would keep for other tires. But, reducing tire pressure got rid of that feeling.
The rear tire did get a lot of wear on the center treads and the treads started to deform and lose some of their sipes. But, nothing abnormal for a lot of use. On the front tire, the center knobs were worn a bit, but the more important side knobs but still kept in good shape with plenty of cornering traction:
|Front tire knobs - all present, but worn in the center|
The tires remain clean and grippy in all conditions - I'd chalk that up to the Black Chili compound. And, the X-King knob spacing is extremely quiet. I have to riding a lot on roads to get to my favorite trails (or linking together long rides between parks) and I never notice road buzz that I would with some other knobbies intended for good grip.
For some of last year, I ran a Race King 29x2.2 ProTection on the rear wheel. The Race King has a lot more volume than the X-King and felt like it added some suspension comfort to my Scale hardtail.
I wish I had a set of calipers to measure them, but the Race King looks closer to what I'd say is a 2.2 width tire (or, possibly even larger) and the X-King looks to be closer to a 2.0 width tire. If you're looking for a very high volume tire, the X-King 2.2 is not for you. I'd recommend the Race King 2.2 as your choice for a high volume tire.
The Race King has a lot lower knobs and doesn't have the same killer traction as the X-King, but it does feel like it rolls faster (the X-Kings have perceptibly low rolling resistance, but the Race Kings are even better). The Race King also felt more supple to me, possibly because I was running it with such low air pressure.
Overall, I'd say my favorite set-up has been the Race King as a rear tire (faster rolling and larger volume) and the X-King in the front for cornering. From talking with others I ride and race with, many others have found this setup to be a great combination too. For situations when I needed better traction or cornering ability, I set up the rear tire with the X-King.
So, in sum, after a year of hard abuse through all conditions and high mileage, I'm impressed with these tires. I'd be impressed with them if I just ended up with zero sidewall cuts (the tires my Continentals replaced suffered front and rear sidewall cuts within 3 hours of use), but their grip, long wear and ability to hold air were really large positives.
5.14.14 Update - I've now been riding the Conti X-King and Race King Protection 29x2.2 tires for over two years. In short, I've had more long-term success with these tires than most other mtb equipment that takes a lot of abuse, like chains, derailleurs, cassettes, etc.. 1) Flat protection - In two years, I've suffered one flat (a cut sidewall on the rocks in Michaux Forest, a location notorious for destroying equipment) that I fixed with a quick hit of rubberized superglue and Stan's. After fixing the sidewall, I rode the same tire for a few more months until I had fully worn out the tread and it was time to replace it with a tire with fresh knobs.
2) Tire life - I can get just about a full season on a front tire and about half a season on a rear tire. I have to ride a bunch of road to get to my local trails. That puts a lot more wear on the rear tire, but I can generally get 3 months out of a rear tire until the tread starts to wear down a bit (I also train a lot on my road bike, so the mtb doesn't get daily use). In rocky terrain, the sidewalls do get a lot more scrubbed up, leading the tires to look like they're at the end of their life, but that's not the case. I've noticed no less air retention from scuffed sidewalls as from a new-looking tire. I think I used the same front tire for a full 9 month mtb season in 2013 to early 2014 when I finally swapped the tire for a new one.