I've tried several different HDR apps for the iPhone, and this app produces the best results; I've received many "wow" comments on photos taken with HDR.
It can automatically identify light and dark areas of an image (you can also select them manually), then quickly,within about a half second, takes two photos and combines them with usually-flawless alignment between the two images. There are four filters to get the HDR effect you want, and you can export the original photos along with the HDR.
As with any two-shot HDR, expect some ghosting if there is rapid movement in the photo or if you can't hold the phone steady (e.g. you're in high winds).