One word: unlock. Until the phone is on Tmo, the unlock is mandatory.
As far as software, for me, SBSettings is vital. Apple still doesn't have a quick way to access Wifi, brightness, airplane mode and other settings, and until they do, I wouldn't think about running without a jailbreak. I also still prefer Lock Info to Apple's Notification Center.
Do their unlocked phones really work on non-approved carriers? I never really looked in to it, it's pretty cost-prohibitive. Also, that doesn't help people that want to buy used last-gen and save a few hundred bucks, or the people who are now off-contract and are still happy with their phone but looking to move.
> Do their unlocked phones really work on non-approved carriers?
yes, obviously. 'unlocked' = 'works on any carrier with which the technology (ie GSM/UMTS, works on the right bands) is compatible'. note that since tmobile almost exclusively uses an unsupported-by-Apple band for 3G (1700MHz), you're getting EDGE at best.
As far as software, for me, SBSettings is vital. Apple still doesn't have a quick way to access Wifi, brightness, airplane mode and other settings, and until they do, I wouldn't think about running without a jailbreak. I also still prefer Lock Info to Apple's Notification Center.