It's tough to really say, since mtb is so multi-faceted. The biggest advance, for me, was in the upper body and grip. I'm not so good with chinups, and you blast every muscle used if you follow XF regularly. When I rode DH after doing it for a while, I had better grip and better upper body fitness, two things I was previously lacking. As for lower body, it strengthens your core and just about every part of your leg, but it doesn't focus on pedaling endurance so much as power. In other words, I'm guessing if you followed it religiously, you'd be able to sprint like a madman, but your long term endurance wouldn't be as good as somebody who was doing spinning 5 days a week.
I definitely loved XF but USE GOOD FORM. I got tired and therefore sloppy and ended up spraining something in my knee. Knocked me out for longer than I'd like to admit, and I haven't been getting back into it.