My brother had an 07 STS. AWD with a Northstar. Thing was fantastic until it hit 100,001 miles. Then it COMPLETELY fell apart. Factory air ride suspension blew out, trans had an issue where it would randomly go into limp mode and not shift (didnt happen all that often, but it happened at the most inconvenient times.) The engine burned oil (yeah, northstar). The seats backs fell off. The front doors were the electronic ones that were activated when the key fob is close. Well the sensor or the solenoid (or whatever it used to open the door) burned out so you could only open the door from the inside or by using the key to pop the door. When the battery on the fob starts getting low, the car wont recognize the fob in the car and will lock you out of the car often and its a PITA to get into because of the electronic door open. All 4 wheel bearings went bad (this has happened on all 4 Cadillac STS's he has owned.)
I could literally go on for days on how much of a steaming pile of shit that car was. I would in no way recommend it.
Most of these problems, had they have been on another car, probably would have been cheap to fix. But since it is a Cadillac and over-engineered (poorly I might add) everything is crazy expensive to fix. Just to replace the headlight the front bumper had to come off (or so they said at the dealer.) I wouldnt touch it and the dealer wanted like $200 to replace a fucking headlight.
Not to mention that the depreciation on them is insane. If you buy one plan buying it outright or keeping it until its paid off.