Can I use the Sprint / Verizon iPhone 5 (A1429) on T-Mobile's network?

Actually, the answer is yes. The Verizon iPhone 5 has been confirmed to be unlocked and compatible with GSM networks such as AT&T and T-Mobile. The only drawback is speed. LTE compatibility is limited on the A1429 iPhone 5 to frequencies compatible with Verizon and Sprint only. Using data on the AT&T or T-Mobile networks with the Verizon iPhone 5 is possible, but will result in slower download speeds.