Phone selection always was a big issue for Sprint, but they have been steadily improving in recent times. Hopefully in the upcoming year, selection and quality will continue to show improvement.
Pricing is also an issue. For example, the new Touch Pro 2 runs $350 on contract. People don't always look at the savings of one carrier vs a carrier for the monthly plan cost, so I'm sure a $200 iPhone looks a lot more comforting in both price and user experience.
The Touch Pro2 is a bit of a specialty item. The HTC Hero, Samsung Moment and Instinct HD are all in the iPhone price range and are marketed as the iPhone's competition. The Pixi will be $100, same as the iPhone 3G old-style.