The only issue is that non of the maps that ship with it heavily use the physics engine. Ageia have shipped two levels that use moderate and heavy physics.
To summarise, the PPU sometimes helps a little on the stock maps, bizarrely in the situation where you're not CPU limited.
On their own maps (go on, guess) they get a sizable performance improvement. Their heavy physics map is not playable without a PPU and barely playable with. Their mid level map is the most interesting. It is playable without a PPU, but is much better with. This is exactly where they should be aiming for. PPU only games are not the way to sell PPUs.
There is a bit of speculation of new and better PPUs in the works. I wouldn't be surprised. Wether they're worth buying depends on wether there's a mass market, popular game out that gets 10-40% gains or more from having a PPU. So far, no, but in the future? Given Crysis is barely runnable and is selling, I don't see why not.