I voted must have..
While the implementation quality can vary from game to game, when it's done correctly, PhysX can make a huge impact in the overall feel and immersion of a game.
Case in point, the top three PhysX games in my opinion are:
1) Batman Arkham City
2) Mafia 2
3) Borderlands 2
After playing these games with PhysX turned on, the very thought of playing them with PhysX turned off becomes almost unimaginable as the games seem utterly bland and incomplete without it.
But, at the same time I also understand that GPU accelerated PhysX will never reach it's true potential as long as it remains specific to one manufacturer, regardless of who's to blame.
Anyway, I think NVidia's strategy is to continue optimizing and refining PhysX, especially software PhysX. That's where the success or failure of PhysX will be determined, and the line between hardware and software PhysX has already been blurred as "hardware PhysX" will run in software mode in the latest games..
It's absolutely crucial that NVidia puts some pressure on developers to stop them from using the older PhysX SDKs. Developers should use the latest PhysX SDKs which run significantly faster on CPUs than the older versions.
Batman Arkham Origins and Call of Duty Ghosts BETTER be using PhysX 3.0 or better..
I think PhysX on medium should be attainable for fast hyperthreaded quad cores, provided the developer uses the version 3.0 or better. Above medium will probably require either spare cycles from the rendering GPU, or a dedicated GPU.