It would make no sense if Nvidia produced drivers for all their cards that didn't work correctly. AFAIK driving multiple monitors does work on newer Nvidia cards like the GTX680. In fact newer cards will not work without the Nvidia web drivers.
I don't know what specific advantage the Nvidia web drivers would have for the GTX570 as when the older versions did work there was no noticeable difference in function or performance compared to the OS X native drivers. The GTX570 is still a very decently performing card especially for CUDA applications where it's similar performance to a GTX680. The downside is it chucks out more heat & needs noisier fans to cool it but it's great value used.
They don't all work correctly. The GTX 570 ( GTX 660 as well ) only one screen works with the Nvidia drivers. Dual screen works with the builtin Mac OS X drivers.