When the 'new' X9 arrived in the middle of last year we jumped for joy, finally we had X.O pattern parts at (semi) sensible prices. The X9 kit found its way onto test bikes without grumbles except from those who didn't like running the long cage on the rear derailleur.
Many XC racers and riders who understand how their gearing really works prefer to use mid, or preferably short cage rear derailleurs, as they allow riders to use a slightly shorter run of chain, with less of it flapping about in the wind. They're also lighter at 212g for the short cage (as opposed to the 230g of the long cage) and less likely to catch on twigs, rocks or narrow rut edges. Finally SRAM heard the call and got busy making short and mid cage versions of the X9 rear mech. But if you've got a long cage X9 and want the short one you'll need a whole new part. Still, there's always eBay for the old one. The cage itself is a composite material and isn't quite the carbon weave bling of X.O but then you're not paying for that either. We like this adaptation to the frankly excellent SRAM X9 a lot and think you will too.