Did the same (MBP --> X1 Carbon) running Lubuntu 16.04.
Few minor irritations - super sensitive touchpad, multi-monitor setup isn't seamless (but works), FN (touch) keys don't all work. Volume keys don't work, but brightness keys do.
All in all, happy with it far. Had to get used to a more keyboard-based workflow rather than touchpad-based -- switching to apps & desktops, moving windows, copy-paste, etc. all now using keyboard.
It’s still a bit iffy. The lack of a proper S3 sleep state is a big, big problem for me. I lose about 6% of the battery life an hour in sleep on F28, and it generates enough heat that I don't feel comfortable putting it in its sleeve while in sleep. Some people have had good luck patching the ACPI tables, but that doesn't work on every distro. I can hibernate okay, but that's really much worse than sleeping.
Other than the power issues, it's a lovely laptop.
I don't regret buying it at all because I don't really mind shaking out problems with new hardware and I usually keep my laptops for ~6 years, but if it's a big deal, I'd buy a used 5th gen X1. Those work flawlessly.
Had both, pretty similar. Better keyboard on X1 Carbon, nicer form-factor on XPS 13, better camera placement on X1 (but I don't use it, so...), X1 is just better. This is using Fedora, you need a later kernel for both of them (4.17+) to stop bad things happening.
