Well a question like that on an open forum is like asking if the Yankees are the greatest team in baseball ... you'll tend to get different answers from different fans many of which won't fall into the realm of "constructive opinion" you asked for.
Simply stated, there's no reason to lose anything..... thousands of ATI users are enjoying PhysX and finding it well worth their investment. I don't consider myself a real gamer but will play about 2 or 3 games per year. With three teenage sons, I do see a lot of games. I can say that I was a lot more impressed with PhysX in Batman than anything yet that I have seen w/ DX11. THG writes:
http://www.tomshardware.com/reviews/batman-arkham-asylum,2465-11.html
Turning on PhysX isn't necessary for gameplay, and you'll never miss it if you don't see the effects. However, when PhysX is enabled, it adds superlative nuances and really creates some “wow” moments. The chunky explosions, cloth effects, paper, fog, and environmental detail enhancements are very cool.....the eye candy is a lot of fun to watch. Once you've turned it on, it's not something you'll turn off if your hardware can handle it.
But don't let THG's, mine or anyone's opinions sway your decision....decide for yaself by popping over to the link below and watching the video:
http://www.firingsquad.com/hardware/batman_arkham_asylum_physx_performance/page2.asp
Now if ya still reading at this point, ya have two choices .....
1. Wait a month and see what nVidia drops .... worse case you'll probably pay a bit less for ya 5970 with DX11 competition on the scene. If ya get a fermi thing, no PhysX worries.
2. If ya stick with the 5970, read this link which in turn points you to the driver hack site which lets you install ATI and nVidia drivers side by side as long as you have Windows 7:
http://www.tomshardware.com/news/nvidia-ATI-physx-patch-gpu,8786.html
http://www.ngohq.com/graphic-cards/16223-nvidia-disables-physx-when-ati-card-is-present-110.html
I gotta assume that with your eye on a $650 5970, the $65 investment in a dedicated PhysX card will not be an issue.
As you will see in the 1,100 post thread referenced in the above article, it works with the latest drivers