We have an XD-V70, 12 channel system that we use at least weekly, with typically 6-10 of the transmitters in use during a Sunday morning (occassionally fewer or all 12); we have been using re-chargeable AA batteries for the last 3 years, and they work perfectly.
While at home we use Eneloop rechargeables, for the church we decided to try the much cheaper equivalent Tenergy brand rechargeables. When I first got them (with three 8-slot 'smart chargers' so I can have them all sitting in the chargers when not in use), I tested the life of the batteries. The typically run-time was ~8 hours, with the shortest being ~6 hours and a few a bit over 9 hours. Just be aware that because of the 'base' voltage difference the 'indicator' on the transmitters of the time remaining is not terribly accurate; they are calibrated for alkaline, not rechargeables.
Over the course of 3 years I've probably replaced a bit over half of the batteries; the 'smart charger' indicates when they no longer hold a charge ... I consider that a great investment. On the belt-pack transmitters some of the batteries are a bit snug and have to be 'shaken' to get the battery out far enough to grasp, but that hasn't been any significant issue.