On September 10th, I received my X1C7 (Gen 7, with 10th gen processor) shipped from Lenovo, and this is a brief review of having used it as my primary computer in the last 3+ months.
Setting up Linux
Linux kernel 5.9 or later has the best hardware support. Everything including Thunderbolt and fingerprint reader works on Linux. I was surprised in particular to see that 5k resolution worked in LG Ultrafine 5k, which is a retina-quality Thunderbolt monitor designed specifically for Macbooks. Compared to previous Thinkpads (such as P71), the trackpad is as good as that of a Macbook.
WiFI can be unstable
The only annoying issue with the Carbon is that the WiFi card included in my laptop experiences periodic disconnections on Linux. Others have reported the same. See X1C7 WiFi issue for details. In the end, I resorted to using a USB wifi dongle to sidestep this problem.
Performance is good enough for general use and programming, but not ideal for heavy lifting. See X1C7 - Moderate Performance for details.
I did not explicitly measure battery life on this laptop with a 4k screen. It looks to be around 6 hours which is more than enough for my use cases. I used the default nixos-hardware configuration; and reddit has some tips. A fellow programmer reported 5 hours.
My next computer, if I choose to buy one in ~3 years, would likely be similar to the X1C7 but with a bit more performance (assuming battery life does not suffer); i.e., if I were to make this decision again, I’d consider Thinkpad X1E or P1 1 - but with integrated graphics (nvidia has poor support on Linux). That said, I still use the Carbon as my primary computer, and use VSCode remote to shift much of the develoment heavylifting to the P71 workstation at home.