I've finally ridden enough on the Superfly 100 to spew my thoughts on the 2x10 drivetrain. I know this isn't about trail building or advocacy but, hey, it's about bikes and how they move so I figured what the hell, this blog can use all the editorial content it can get.
My experience is on the Sram X0 2x10 drivetrain with a 39/26 in the front and a 12/36 on the backside. At first I wasn't sure what to think with all the hype about no dropped chains, no chainsuck, you'll ride faster, chains don't wear out, you don't need a chain, whatever the hype machine had to say in Mountain Bike Faction.
At Ouachita I dropped the chain off the large chainring twice ... and twice ripped some sweet metal flake paint clean off the drive side chainstay.
In Taos on the steep South Boundary descent I dropped the chain off the big chainring and pedal/jam-sucked the chain causing the 10 speed chain to twist a link. I didn't know why it shifted like a worn out a VW Bus until the next day while cleaning the bike.
Other than those two semi-major incidents of droppage the system has been terrific. The shifter pods took a bit to get used with the upshift action being another thumb-activated push action. Unlike the Shimano system you press a smaller paddle just behind the main thumb downshifted. The shifting has been crisp and tight; on steep climbs the rear derailleur moves easily and deliberately when the going gets tough.
The gearing has been the best part. Unlike a 3x10 system where you learn about the badness of cross-chaining or how lame you are to use the granny or wonder why the big ring exists because it's rarely used when you're ACTUALLY mountain biking on singletrack, well, the 2x10 system condenses the gears down into a much more useable range. I found myself riding FASTER because the gears matched closer together and I didn't have to think about the shifting - there's no cross-chaining voodoo and the larger ring is used more often than not simply because the gear mavens have offered use a pizza wheel 36er to get us up those steep climbs.
After riding a 2x10 drivetrain most of 2011, I am convinced I won't be going back to a 3x10 ever - unless it's on my existing bikes. What I mean is that I won't be buying a new 3 ring crank in my future, not with our easier Missouri terrain. And to be as minimal as possible, I would say most people would enjoy a 1x10 with the newly available 36 hanging on the back. Those two extra teeth can conquer worlds… well, almost.
While I wouldn't say you should just go out and replace your perfectly good system, I would say that you should investigate the 2x10 options out there offered by Sram and Shimano if you are building up a new bike or have come to the conclusion that your current drivetrain is played out. I've checked complete systems and they do have that sticker shock going for them, however, the new XT keeps the price within reason and the x7 or x9 is supposedly pretty good stuff.
All of the above ramblings are solely mine and I don't take responsibility for your knee surgery should you discover the 2x10 setup is not for you. However, if you have any XT or XX product laying around I might be interested in applying it to some bikes for further testing.