Currently, the only smartphone to take advantage of the fast network is the HTC EVO 4G. However, the Samsung Epic 4G's release is just around the corner. Using these devices requires a few preselect plans starting at $69.99 plus the $10/month Premium Data add-on for 4G usage. Sprint is offering money back guarantee for trying out its 4G speeds, and will reimburse the fees if you are not satisfied after the 30 days trial period.
HTC EVO 4G Specifications | Review
Samsung Epic 4G Specifications