Its basically just like an o-ring chain, except the cross section of the o-ring looks like an X. That is, looking at one of the o-rings, it looks like it has little groves around the circumfrence. Cut it, look at the end and sure enough it looks sorta like an X.
Both O and X ring chains rob a little more power than a standard chain. Neither will be enough to notice on a KX250 and certainly not on a KX500. The X ring will rob modestly less power than an O-ring chain. On my street bike (which i commute on, so exact same route everyday year round) I consistently get 1-1.5mpg better fuel mileage with an X ring than an O ring chain. No noticeable power difference (but the bike has 120wrhp) but I assuem the lower drag of the x-ring chain is responsible for the increased mileage.
On an X-ring lasting longer, I haven't worn one out yet, but best I can tell it will last longer. I still have O-ring chains on my dirt bikes, but when they wear out I will replace with X ring chains. My street bike was getting about 20k out of an o-ring chain. I have a bit over 10k on my x ring now and I've adjusted it once.