SRAM uses a sporadic mix of hex and torx fasteners throughout the X1 group, even on its individual components. People have happily been mounting derailleurs, for example, to frames with 5mm fittings for as long as I’ve been riding bikes. SRAM uses a T25 fitting, and I’ve already heard of a number of them stripping (be sure to grease your mounting bolts).
I can think of no good reason to use torx fasteners in a relatively low stress application where the dimensions of the bolt head don’t really matter. Beyond that, scattering torx fasteners around my bike amongst standard hex fittings means I have to carry a set of torx wrenches and a set of hex wrenches with me on a ride, and it would be nice if this wasn’t the case.
That gripe aside, I really don’t have any complaints about the X1 drivetrain.
I’ve used the X1 kit on about 35 rides, which certainly isn’t enough time to arrive at any conclusions about long-term durability. No part of the system seem chintzy or flimsy though, and I expect it to last as long as any other comparable component group.
In a more general sense, my experience has always been that higher end components tend to last longer under normal use. Higher end shifters tend to use bearings instead of bushings, and all the components’ mechanisms are made with more durable metal and less plastic. Time will tell if the X1 holds up as well as the XX1 or XO1 options.
If SRAM’s 10-speed groups are a comparable indicator, then I will be particularly interested to see whether the X1’s shifter starts to feel a bit gummy after a season of riding, and whether the derailleur slowly develops a bit of slop that takes the crispness off of the shifting. The clutch mechanism in SRAM’s X9 and XO derailleurs can also get stiffer over time, presumably as it internally gunks itself up, so we’ll see if the X1 fares differently.
11-speed, single-ring drivetrains are quickly starting to ruin my taste for 2×10 systems, since 11-speed setups are simpler, cleaner, and lighter.
So if you’re looking to convert your setup to a single-ring, 11-speed drivetrain now, and you like what you’ve heard about the way the X1 system performed, I don’t see any reason not to consider the X1 group. The system shifts better than a 10-speed X9 setup, while being quite a bit lighter and only moderately more expensive. And while in some respects the X1 kit isn’t quite as nice as the top of the line XX1, it doesn’t give up as much performance as you might think given the price difference between the two groups.