Quark sure got some customers from Fenix & Nitecore but at least Nitecore is actually quite different, so it all depend on your needs and preference.
The whole Quark Lego design is good for people like to play and switch between different configurations, batteries, and that make the Quark a little cheaper. They have latest LED, lots of accessories, and some people like the "lower low".
But they are not my favorite, because I want my flashlight solely in AA batteries, while Quark's shared head make the Quark AA not very good looking. And the driver came from fenix so have the preflash. Some people said that's for efficiency to avoid parasite drain, and you can put it on pants before you turn it on, but I'll never make comprise like that if I have better alternatives.
Besides, none of the regular or tactical UI appeals me. I don't want to scroll several modes before I reach the mode I want, and even you can choose any mode you want in 2 presets, how can that 2 preset enough? You always ends up reprogramming. It's same with Jetbeam IBS. It's fun that you can program any modes you want and you have more modes than tactical Quark. But still, imagine you need some mode you didn't program before, you still need to reprogram, and I don't think that is a easy and quick job in a hurry.
So far, I think Nitecore D10 SP have a very good UI, you have memorized brightness, you have SOS/strobe tucked away and immediately accessible AT THE SAME TIME. Zebralight H501 have a good UI too, you can get certain mode in one action.