Nice idea - shame about the battery
This is basically a sound box, and so works the same way the open body of a guitar or violin functions. The iPhone makes the device resonate and so makes sure the maximum amount of energy is turned from speaker/iPhone casing vibration into sound waves.
The problem is that this is driven by the iPhone speaker, and so will use (esp at high volume) much more battery power than is needed to produce the small voltages used as a signal by powered speakers. It will probably also lack in bass volume, and will never be that loud, so the iPhone volume will need to be at max most of the time. Considering how short the iPhone battery life is, you'll only want to use it with the iPhone plugged into the mains... in which case you might as well use mains powered speakers.
I'd say this is a neat stocking-filler for the hard-to-buy-for iPhone owner, or is a fun gadget for technology show-offs (which fits in with iPhone owner demographic quite nicely). As a practical way of listening to your music, I'd suggest this isn't worth the money.