Since most of my free time since I got my GT2 a month ago has been devoted to reading posts on this site and toying with it, these are the issues (some have had, many have not) I've noticed that you should be aware of:
1. The sunroof may have some creaking over time, as many from all manufacturers do. There are a number of threads devoted to fixing this and a large consensus is focused on adding washers or padding where Kia likely should have done this themselves. I haven't heard of someone not being assisted by Kia when asked, though results are mixed.
2. Brakes may (pads, possibly rotors, not calipers) have some wobble when braking due to inferior quality pads being installed on American builds. Kia seems to be working with folks on fixes that have issues but for those with issues, the best solution is to buy decent after market pads for at least the front, possibly the back. I'm almost 2k miles in without issues and drive spiritedly maybe 10% of the time.
3. Leather may show wear sooner than you expect. Mine has a few very light crease marks, but nothing crazy. I plan on treating the leather shortly as I believe it sat on the dealer's lot without a proper clean/condition. I have red leather, love it so far, and expect to see minor wear sooner given the lighter color.
4. It's a GDI engine and will thus have carbon buildup in the intake sooner than non GDI engines. These engines are as good as ever and I highly doubt you'd see a lot of buildup before 40/50k miles. That being said, oil catch cans are easy installs and I installed one. This is my only planned, "mod," as the car runs great otherwise in my opinion.
5. Not even sure I should list this as it's a GT car but many dislike the sound or lack thereof. I enjoy taking long rides and the wife despises unnecessary sounds. I think it sounds pretty good when you floor it so I'm all around very happy. There are a plethora of ways to add exhaust and turbo noises to your heart's content.
I've had no issues thus far and it's bar none my favorite car to date. In my adult life I've had a Scion FR-S, Honda HR-V,
Buick Regal TourX, and now the Stinger. It's literally like all of my previous cars combined with AWD, insane utility (folding seats/hatchback), luxury, and an engine/drivetrain that's stupid fun to drive. If you're on the fence I'd say do it; I've never seen a car community of this size so into a car, not to mention generally kind and responsive to any questions/issues.