I use one (2015 model), it's ok. The 2560x1440 screen works well (using 1.5 scaling) with unity, a bit less so with cinnamon, as it Cinnamon only supports integer scaling. Chrome has a scaling flag that is independent, that's a bit extra work. I regularly connect an external 4k screen over DP, works fine and runs at 60Hz.
The touchpad is ok, but wish it was larger and dealt with palms better. Over time I adapted and now am careful to not touch it while typing (or maybe some update fixed it).
I use an older Ubuntu version and there are a two bugs - it doesn't always lock when closing the lid and the touchpad is sometimes disabled on wakeup. Switching to a terminal (ctrl-alt-1) and back reenables it.
Overall, I'm happy - great keyboard, decent HiDPI screen and all in a very light machine. 16GB RAM and longer battery life would be nice, but I'm happy with the weight tradeoff.
Fairly hassle-free. I have both the 2015 X1 Carbon (with Debian Jessie) and 2016 X1 Yoga (with Debian Stretch). I needed to install an extra xserver-xorg-input-* package for the touchscreen on the latter to work nicely and Firefox Aurora to get gesture support, but everything else just works.
The keyboard on the X1 Yoga is sort of meh, though.
