The premise of physics acceleration by dedicated hardware is a solid one. While there's always a downside to adding an extra piece of hardware to a PC to do something better - cost, noise, heat and power consumption - the upsides usually make it worthwhile.
However, much like the early days of 3D graphics, the PhysX PPU needs traction with a killer title or two to make the world sit up and notice. GR:AW isn't the one, and neither is Bet on Soldier or Rise of Nations.
I remember the feeling the first time I fired up GLQuake on my Voodoo card. I get almost the same feeling when I fire it up now. I don't get that feeling when I play PhysX-enabled titles - the effects presented aren't overwhelming and certainly not persistent.
Developer support is pretty strong, though, with Unreal Engine 3.0 the biggest current proponent. There are over 20 PhysX-compatible titles in the works.
Technically, the API needs to mature to encourage developers to make the investment, and if you are making it you have to think hard about its integration, lest you run into problems with the latency of return data when using it. GPU effects physics - just Havok FX right now - is the easier bet for a developer looking to tack effects-based stuff onto an existing engine, especially if they already license Havok's physics API.
Outside of gaming there's some scope for the PhysX hardware to be used for general purpose parallel floating-point computation, but the API simply doesn't cater for that in any meaningful way just yet. The interconnect it's sat on somewhat limits its usefulness in that respect, and it only supports 32-bit Windows at the time of writing. The goodly chunk of on-board storage counts in its favour for such applications, though.
The limited number of titles and their disappointing use of the PhysX PPU means that, currently, there's no reason to spend the £200+ to acquire a PhysX card. The current effects in the supported games aren't worth the price and potential performance drop. Cell Factor and awesome Unreal Engine 3.0 games, where art thou? Without them, the PhysX hardware is merely a curiosity. But one to watch.