I upgraded from a reference GTX 980 Ti due to my old card dying.
Performance is noticeably better, but not really worth the price. The 980 Ti is still a very decent card by comparison, even in 4k.
The biggest improvement over my
reference 980 Ti is the noise and cooling. The 980 Ti would very quickly get very hot under load. It didn't perform quite as well as it should have (according to benchmarks), likely because it quickly hit its target temperature (83°C) and started throttling. Trying to get the fans turn faster improved performance, but also made the card very loud. At 110% power target and with sufficiently fast fans, the card would come close to a 1080 at the cost of extreme noise.
Enter the MSI 1080 Armor.
On idle, the fans don't spin (as opposed to the 22% minimum fan speed of the reference 980 Ti). The card still manages to run cooler than the reference 980 Ti.
Under load, the card would never reach 80°C and still remains very quiet. At 70% fan speed, the card is still very quiet. At 100% fan speed, it gets a bit loud. If you wear a headset, you probably won't hear the card, even at 100% fan speed.
Even though it was quite expensive, I don't regret buying this card. The better performance wasn't quite worth the price, but the significantly better cooling makes up for it.
If you're looking for a silent card that performs well, buy this. If you have a 980 Ti and are very satisfied with it, wait for the next generation.