First off, PhysX isn't that only physics engine available.
There are hundreds of games with their own physics engine or uses a third party engine like Havok. Among those games, the ones that uses Nvidia PhysX is only a handful while the number of good ones is pretty much only batman.
Second, physics done by GPU isn't any more efficient, realistic, or accurate. It is all number crunch, more is dependent on the engine features. But you do lose out on GPU performance when running Nvidia PhysX. While quad core CPUs have a lot of threads to go around.