Initially, there was an issue with S3 sleep but Lenovo addressed that with a firmware update. Lenovo also recently joined the Linux Vendor Firmware Service (LVFS). It is now extremely easy to update firmware in Linux by executing the fwupd command. The one remaining issue is some thermal throttling threshold issue. I've installed a service that is a work around but I'm hoping this gets addressed in a subsequent firmware update.
I was coming from a MBP as well and I've completely switched to Linux. I went back and forth between getting the Dell XPS or the X1. I finally settled on the X1 because I liked the keyboard more after using both at Microcenter, but it was a tough decision.
The Arch wiki entry[0] on the X1 is a good reference point regardless of what distro you use.
[0] https://wiki.archlinux.org/index.php/Lenovo_ThinkPad_X1_Carb...