OK, I know that 2x9 isn't a new idea at all and technically 2x10 isn't that different but the extra sprocket does mean that running a double up front rather than triple doesn't mean having to lose a significant amount of gear range/having big jumps between gears and means less shifting at the front and more useable gears than a triple.
XX looks great and has had a lot of work go into it but it's going to be silly money. I quite like the idea of 2x10 (having seen plenty of cross bikes do just fine with 10 speed in some horrible conditions without trouble) and I was wondering how easy it'd be to set up a 2x10 for mtbing without spending XX money or waiting for it to trickle down.
The obvious problem is a 10 speed shifter but quietly, Shimano already do this so that you can use road components with a flat bar for hybrids - there are Ultegra and Dura Ace level versions with the Ultegra one (R770) costing about £40 for a RH one alone.
The chain isn't an issue as road ones are basically the same as mtb ones.
Then the ratios.
XX is due to have a 11-36 cassette to give a similar gear range to a triple with a more racy 11-32 option also available but the range
They also suggest having front rings in the ration 1:1.5 as this gives the most shifting windows (basically every second tooth on the smaller ring matches the third tooth on the bigger ring). It's hard to say if this has a significant benefit but it seems logical and I'll be interested to try. Ratios they're going to do are 26/39, 28/42, 30/45
So, I've been thinking of 24/36 for chainrings (a bit smaller than XX for reasons of the cassette as discussed below but easy enough to get hold of and I reckon a steel 24 will last fine).
For the cassette, it's a bit more of a faff. Though you can buy 11-32 10 speed cassettes (IRD do one), they're really expensive. There aren't currently any cassettes with a 36 out there though I believe that next year, there will be one designed for 29ers though it's only 9 speed.
A much cheaper solution is to buy a 11-28 10 speed road cassette (£40ish), split it, remove one sprocket (the 12) and then add in the top 32 tooth sprocket from a 9 speed mtb cassette (plenty of people have these in good condition on a worn cassette). Job done.
So, for comparison, the ranges of various setups:
standard 22/32/44 triple with 11-32 cassette: 0.69 to 4.00
XX 26/39 with 11-34 cassette: 0.72 to 3.55 (losing the very top gear from a std setup)
My setup above - 24/36 with 11-32 cassette: 0.75 to 3.27 (losing approx the top two gears from a std setup)
Just my musings and I can't really decide if it's worth it though if I did have a bike with a knackered drivetrain I'd be very tempted to try.