I don't think it's worth it to buy a card just for that, but if you have an old geforce card lying around that has the abbility to be a physX card (8xxx-9xxx series if I am not wrong), then it would worth it for the few games that support it.
Its all about Visuals as far as i can see. Its like the DX10 thing all over again. Yes its pretty but sometimes i get the feeling that the effect isnt needed or sometimes it just looks wrong, as an example its things like dust kicking up on a floor, way too much dust and in a neat ish circular pattern, just looks wrong and not needed to me.
Thats not to say all the implimentations of it are bad, some look quite good. Would it make me but one card over another though ? No.
It could have been good, as it was able to do a lot of physics processing, but it was costly and few games did support it. Their physics cards could have been good if it was able to do other things (video transcoding/rendering/etc) rather than just be an (expensive) add-on to gaming.
I concur with the statements being echoed, PhysX is hardly the leap forward that 3DFX Voodoo 1 was for 3D graphics was back in the mid/late 90's that some people expected it to be. Now the big N is behind it PhysX is getting a lot more support from developers but I've not yet seen or played a game where it has proved it's worth.
The trouble with PhysX it lacks that killer title that can really push it to the masses, look at the games that support it (google it) and the titles are hardly ground breaking classics. To put it another way, you could play a game through from start to finish and you probably wouldn't notice if the game was using PhysX or not.
You have to remember PhysX isn't the only standard out their, Havok is big competition (now owned by Intel and back and supported by ATI), whereas PhysX can run on Geforce 8000 and 9000 cards Havok can share the work load between CPU and GPU accordingly and uses Open CL which is great for future support (at least in principal). As well as Havok a lot of studios prefer to use their own in house Physics solution such as the engine found on Empire Total War.
Now the big N is behind it PhysX is getting a lot more support from developers but I've not yet seen or played a game where it has proved it's worth.
One thing I have learned from all these "advanced tech" is that developers adopt what is applicable to most systems. PhysX may provide insane performance improvements but requires nVidia cards which is a deal breaker compared to Havok. That is where they shine, as Havok's physics could run on any computer with a cpu (which computer doesn't ).
This behavior is understandable as developers need to learn only one thing but it applies to more systems (less work, more gain).