One maintenance item that Cessna updated recently is to add eddy current inspection of the fuel panels instead of just a visual inspection. Corrosion of the fuel panels seems to be fairly common. I don't know how widespread it is, but corrosion extending into the wings has been an issue on some Xs. I'm not sure of the fix on that, but if I were buying I'd be looking closely for that.
A couple of complaints I have with the X is that I think the air cycle machines are a bit weak for the plane. It can be hard to cool down - the cockpit, anyway. Also, at ISA + temps, the plane is a slow climber with the anti-ice protection on.
I have only flown 2 Xs and they both had/have a high dispatch rate. Having a full time mechanic, though, is key to that.
I think overall it is a great plane and I love flying it.
Let me know if you have any specific questions.