The iPhone SE fills a hole in Apple's product lineup on the low end -- that is, Apple's low end. It's priced lower than Apple had been pricing its 2-year-old model, but it brings the capabilities of the iPhone 6s, which play toward Apple's service business (e.g. Apple Pay). Ultimately, the iPhone SE is about bringing more customers into Apple's ecosystem and feeding its services business. Doing that would qualify it as a success.