I do not have an iPhone, so I cannot personally vouch specifically for the quality of this app, however, the creator of the app also developed the Tefillon app which I use on my Nokia phone. The Tefillon is a great siddur application for mobile phones that i use whenever I am davening somewhere with not enough siddurs or in ad hoc minyanim. Based on the Tefillon, the Megillah app is probably pretty good.
This is an exclusive & free cellphone app (developed in RBS!) specially for Purim!