Despite falling short of some expectations (I'm particularly disappointed by the environment texture quality), Witcher 3 is still without a doubt one of the best-looking games ever made. The detail is dense, the art design is magnificent, and there's an impressive amount of physics on display.
When it comes to hair, Witcher 3 is in a class of its own. Think Tomb Raider's TressFX but less showy, and applied more frequently. Geralt's hair is simply the most natural-looking hair you've ever seen in a game, and the fur on some of the creatures is just fantastic. Of course, all this hair (up to 125,000 strands on-screen at a time, apparently!) comes at a performance cost, and unfortunately, it doesn't use PhysX. Witcher 3's hair physics uses Nvidia HairWorks, which uses your main GPU only.
Witcher 3 doesn't use PhysX for hair, but it does use it for cloth and destruction effects. These effects are used on all platforms (consoles too), and are said to use the CPU. However, I can confirm that they do utilise my dedicated PhysX card.....though not much - the most I saw was about 9% usage.
Shut up and test
CPU: core i7 4770k
GPUs: Titan SLI (x2), GTX650ti for PhysX
Driver: 352.86 ("Witcher 3 ready")
I went to areas where there were a lot of PhysX enabled objects (flags, banners, hanging herbs, etc.) and tested the framerate both while standing still, and while using the Aard sign to fling everything about. I then averaged the results below.
Without dedicated PhysX card: 50.1
With dedicated PhysX card: 50.8
Yup. Not very impressive at all. I'll add that I don't think the 0.7fps is mere statistical noise. I really did notice a ~1 fps difference in most of the areas I tested in, especially when throwing Aard. But 1fps is 1fps, so who cares, right?
I'll also add a caveat: I'm not very far into the game. It's entirely possible that there are some areas I haven't seen that are much more demanding for PhysX (This happens very often in PhysX-enabled games). It's also entirely possible that with a levelled up Geralt who possess a much stronger Aard, I'd see much larger and more taxing destruction PhysX effects . So, while I'm not flipping somersaults about my 0.7fps increase, I am at least content to know that my PhysX card will be ready to absorb anything the game might throw at it later on.
So the upshot is this: Witcher 3 ain't no Metro: Last Light. If you already have a PhysX card, keep it in - who knows, it may save you the occasional minor slowdown. But if you don't, don't go buying one just for Witcher 3.