We use the iPower 9v 520mAh LiPoly batteries at work in wireless mic packs all the time -- and they work really, really well.
They operate at 8.4v (like most NiMh rechargeables also do), but last about 2x as long as the NiMh equivalents.
Some devices that are calibrated for alkalines will read "low battery" when inserting a fresh iPower because of the 8.4v, but they still run a long time.
I'm not sure a 7.2v battery would function well in a lot of 9v applications, but I could be wrong.