Watched it and liked it. Those mongols must have been terrifying when they went into battle. Horse archers were one of the greatest combinations ever created for war.  
 
Now we just need some some slinging videos!  

Have you seen this one Little? Imagine having several thousand of these guys running circles around ordinary troops ripping them to shreds with their arrows. Crazy stuff.
 
http://www.youtube.com/watch?v=2yorHswhzrU&feature=related

fascinating. I really should put some research hours into learning about the mongols. They don't enough credit in most history books.
 
I don't know about nomadic being better than sedentary. Because if we learned one thing from the american civil war it is that sedentary armies can live off the land for a time too. But there's always a danger in that particularly as your army gets bigger. Sherman had maps from the last census and so led his men through the richer counties (ie the ones that actually had enough food to support his army). But he knew that if he stopped, he would exhaust the local resources. I don't see the mongols being free of that issue either.  
 
I think this is a "which is better a hammer or a wrench?" type of question. It depends what you want to use your army to do.