I had one with an i7-8650U CPU -- Intel 8th gen core which I think makes it a 6th gen X1? So not directly relevant, but ...
I liked it as an ultra-light laptop for general web browsing, email, writing and compiling and testing small programs. The battery life is good. I liked the backlit keyboard (but not as much as Apple ones). Running Ubuntu sleep and wake was actually reliable .. wow.
I have a NUC with exactly the same i-8650U CPU.
On large CPU-intensive tasks the NUC is 50% faster than the laptop. Both would start an all-cores task at 3.4 GHz but the laptop *very* quickly fell down from that to eventually around 2.0 GHz. The NUC stayed at 3.4 for several minutes and then over the course of half an hour gradually fell to 2.8 GHz, but never lower.
After, say, 30 minutes, the NUC was running 40% faster than the laptop (and stayed that way), but the NUC stayed at or near 3.4 long enough that on tasks between a couple of minutes and 30 minutes the average advantage was bigger.
They both coped with single-threaded code at 4.2 GHz (or near enough) just fine.