If I was an iPhone user unless I desired and had LTE coverage, this would have to be at least an option. The possibility of saving $70-$80/mo. and being able to recoup money from previous iPhone by selling it would negate any up front cost of the phone or a lock down to carrier.
This is assuming trial service is satisfactory if not then it's not an option.
$000.00 Virgin Mobile iPhone cost
$000.00 - current iPhone (sold)
$000.00 divided by monthly savings $00.00 = # of months until free