At its most basic, the Phocus is an iPhone case that lets you attach the included wide-angle or macro lenses for souping up your photography.
And the lenses work well. I found that the macro lens let me get extremely near a subject for close-up photography, less than two inches away. An unassisted iPhone can't photograph anything at that range without monstrous blurs.
The wide-angle was equally useful, letting me compress huge shots into the phone's field of view.
My only complaint about the Phocus is that it fits the iPhone snugly enough to pry up any screen protector you might use. If you don't use one, then obviously that's not a concern.