It's fine for the occasional speakerphone call or voice memo, but if you want to record a concert that's worthy of sharing, you'll want to upgrade your iPhone's mic with Tascam's iM2 which adds a set of professional stereo mics.
Besides being just a mono affair, the mics built in to the iPhone, iPod touch and iPad are designed for use in quieter settings, like sitting on the couch and FaceTiming with a friend. When used in a louder environment they'll distort, so in addition to a stereo set of condenser mics, Tascam has included its own pre-amp in their iM2 allowing it to handle sounds as loud as 125db. And if things get even louder, there's an adjustable input level control letting you dial things back.
The mics can rotate 180 degrees front to back, depending on how you hold your device and who you're recording, and it features a USB pass-through allowing your device to be charged while the iM2 is attached to its dock connector. It is unfortunately another powered accessory that will take its toll on your iOS device's battery life, but as a consolation prize Tascam is including a free PCM recorder app with the $US79 mic. I know, not really a consolation at all, but no one ever said the life of a concert bootlegger would be an easy one. [Tascam via AVWatch]