This is just one guy's opinion, so take it for what it is. I really, really hate Continental Race Kings. Sure, they're fast. Around the 16km track that I used to train on I got my best times with them. But, and this is a big "but", they have very little grip. In fact, I'd say they have worse grip than my lawnmower's plastic wheels...and at anything below 45psi they'd pinch flat virtually every time I ride. So while they're fast and easy rolling, I didn't have fun on my bike. At all! You have to take corners carefully, because they wash out without warning, they're rubbish on technical bits, and if you're not careful you'll be changing tubes alot. The Conti's recommended 50psi, they're rock hard, which makes them rattle your teeth out, if you're on a hardtail. I also hate the fact that they're not as wide as Continental claims. The 2.2 is generally narrower than most 2.0s so if you think you're getting a fatter tyre, not so. In short, if you're only interested in XC racing on smooth dirt roads and mostly hardpack, get race kings. Otherwise stay away. For about a half of the price, you can rather fit El cheapo Michelin country dry's. I ran those at 30 psi with no problems, they roll pretty quick and they have mucho better cornering grip. They suck in the mud though, but then again so do race kings. My favourite tyre thus far is the Kenda Nevegal. They're 2 minutes slower round my 16km training track, but I have fun riding them. You can clamber up stuff, take corners hard, power through mud, and they just don't give up their grip. Rocks, mud, sand, snow, glass, whatever, bring it on. They are hard to peddle though, because they don't roll smoothly, and many people hate them because of this, but in the end you get a better workout and I think you have more fun. I'll probably try Monorails next. Their tread pattern makes sense to me. Small tight knobs down the middle for easy rolling. Fat knobs on the outside for cornering grip. But thus far, I love my Nevegals. Oh and another thing, because Nevegals are often fitted standard on bikes, and most people swop the stock tyres out for Race Kings, you can generally get Nevegals cheaply if you can find 'em.
