See the XTS section for more details, but you are in a great position when it comes to "things to watch for". The XTS is about as problem-free as (current) premium cars get. You will have to deal with a dead CUE screen at some point, and likely a strut or two. You might have to replace the air-ride compressor. Other than that it's the standard 4000lb premium luxury car stuff - weird tire wear, wheel damage and balancing, hubs, warped rotors, and the occasional electrical glitch that you fix by letting the car sit overnight.
So drive and enjoy!