The Digitech iStomp isn't the first iPhone-controlled guitar effects pedal, but it's the first one that feels more like a legit guitar accessory than an iPhone toy.
Like Digitech's massive iPad pedal board before it, the iStomp's design is a cut above the rest because it's conceived with guitar players in mind. The important difference lies in how the hardware uses the iPhone. On other units, the digital sound processing happens on the iPhone, whereas on the iStomp the effects are actually loaded onto the device itself. From there, you don't need your iPhone again until you want to change the effect. Guitar players are going to love this because it's a lot like using any other effects pedal when the iPhone is disconnected.