I can't speak to this generation of the machine, but I've had a decent experience with Ubuntu on the 3rd gen Carbon X1. I have constant issues with the wifi, though... networkmanager needs to be restarted a couple times a day or else the OS will just lose connection and say wifi is disabled. There is also a general sluggishness, especially when web-browsing, that I have not yet tracked down.

All in all, though, it is fine, and the hardware is very nice. I did have trouble getting any other distros besides Ubuntu to install and run properly, though, YMMV.