Windows works very well. I had issues with CPU overheating when playing Witcher 3, but I reduced the voltage on CPU a little without reducing clock with an Intel program and it succeeded in reducing temperatures without stability issues.
On Ubuntu (https://github.com/stockmind/dell-xps-9560-ubuntu-respin) there was some fun in making sure that Nvidia GPU is off during normal work in order to reduce battery usage. I can get GPU back on with single command if I need to do some simple machine learning. Right now during my normal working (Firefox with 200+ tabs, Chrome, VS Code running) with 100% battery, powertop gives me a discharge rate of 12.3 W and estimated remaining time of 7 hours, 31 minutes. It normally is about enough for a full workday.
The Bluetooth sometimes goes off after closing the computer (suspend) while being connected to headphones. In order to fix the issue, I need to restart the computer waiting for about 10 seconds downtime before starting again. Funny enough, right now Ubuntu is claiming that Bloototh is off but it is actually working. There are also very sporadic issues with video. For example, sometimes YouTube will just crash and I will need to restart the system to get it back working.
I'm happy with the build quality (beside mentioned thermal properties). I especially like the sturdy screen. Keyboard and trackpad, for me at least, are about as good as Mac. I liked Mac trackpad maybe a little bit more. IO in XPS is good. I'm certainly missing MagSafe plug (charger plug that disconnects while pulled). I should also mention that when the battery is full and I'm connected to the charger I can hear a very weak coil whine. Kind of like an HDD head working but possibly even less noisy, most time I don't even notice. When I do notice I just disconnect it from the charger and the whine is gone.
I never had to deal with Dell support, so I don't know how it looks like. But I like that a lot of technical people use XPS, so it's easy to Google issues.
Overall, I would recommend XPS. (My girlfriend has one and she is happy with it.)