Just for one thing it seems one can use the iPhone camera one-handed, something which is virtually impossible with only the screen button.
Aluminium and leather, sounds like it'll look and feel good. (Not super-cheap though, about £50/$75.)
It communicates to a dedicated app via the connector.
[Update: I have mine now, and it does not perform so great really. It keeps loosing the connection to the phone, and support doesn't seem to have a solution. A shame.]
They say the fit is very secure and snug, the phone won't drop out. They also say that one can use all the normal iPhone functions while the POPA is mounted. That's good. It would be clumsy to have to take it on and off. And I suspect it will make the iPhone easier to hold too, while still fit in many pockets.
It was originally called RedPop, here's the introduction video on Kickstarter. And a newer FAQ page.
I've ordered one, I'll be interested to use it. I think the new iPhones are great as take-everywhere cameras, but one has to admit they are slightly fiddly to hold and shoot.
Here's a very positive review of the POPA. Among the interesting bits of info is that the shutter is instantaneous, since it does not wait for focus lock. But since the iPhone autofocuses continually, or you can use touch-to-focus, I think this is a good feature, potentially allowing for precise capturing of moving subjects, one of the things which has always been an issue with phone cameras and other compacts. Of course one has to learn to watch for whether the subject is in focus before pressing the shutter. (*Not* having to learn that is the reason compact cameras have that focus wait.)
That is a feature of the app, actually (and the app is free and can be used without the grip). This other review says:
"Yes, I’m still shooting with the iPhone 4, but the amount of time it takes for the camera to focus, un-focus, focus and then finally lock focus has been killing me*. As a street photographer, speed is everything. Capturing moments that might exist for a split second simply cannot be accomplished with a great deal of shutter lag.
Enter the POPA app.
What Brendan and his team did with the POPA app is put the camera on a sort of ‘shutter release’ priority as opposed to a ‘focus’ priority. Which, in turn means … you guessed it … no shutter or focus lag. When you press that big red POPA button, or tap the red POPA button on the screen, the camera fires."
*A little real life research tells me that interestingly, on the iPhone 4S (but not the 4) and when using the Volume Up button as shutter (but not the screen button), you also have an instant shutter, regardless of focus!
And in my testing, the 4S is no slouch at focusing, less than half a second normally, so it's not so durn bad as a street camera just in itself. You have to be pretty hot on that shutter button to catch out the camera on focus.
UPDATE: TCGirl found these better pictures on The Verge (an interesting tech mag, by the way).
By the way, one-handed shooting should only be done in bright light (short shutter-times) to avoid shaken pictures, particularly since the lens is at the extreme other end of the iPhone from the handle.