First it's important to realize that the entire group is new, all the parts have been re-thought and changed specifically for XX. One of the coolest features for me (a bicycle mechanic) is that you only need 1 tool to adjust anything in the group, a T25 torx driver. Before you needed a phillips screwdriver, a 6mm hex wrench, a 5mm hex, a 4mm hex, a 2.5mm hex and your fingers. T25 is the same tool we have been using for disc rotors for years. Now it's for everything including the limit screws, cable anchoring bolts, derailleur clamp bolts, etc. I love it! 1 tool for the saddle bag.
The cassette is pretty spectacular itself. If you haven't seen the new SRAM Red cassette, SRAM's top end Red & XX cassettes are milled from a chunk of steel. The difference is that the XX cassette is drilled out for mud and dust removal. The lowest gear is the backplate of the cassette and is made of aluminum. Yes you guessed it, lightweight. The standard XTR 11-34t (9spd) cassette is 250 grams. The SRAM XX 11-36t (10spd) cassette is 218. Wow.
I'm just realizing this is going to be a long review. Come back later for more on XX!