I setup my vivitar 285HV and tested the PowerGenix NiZn Batteries along with the Sanyo Eneloops 2000 mAh AA batteries and the Sanyo 2700 mAh AA batteries. Check the video for the results of the recycle time on the flash.
--link to video
They have about 30% more capacity than NiMH
runs at 1.6 V instead of 1.2 which should provide faster recycle times with a flash
GREEN!!! NiZn batteries use no heavy/toxic metals (mercury, lead or cadmium), or metal hydrides (rare earth metals) that are difficult to recycle. Both nickel and zinc are commonly occurring elements in nature and are not very hazardous. Zinc and nickel can be fully recycled. - ( wikipedia.org quote )
They actually weight LESS than a NiMH battery
You wont get as many flashes out of these batteries.
You NEED to buy their own special charger i recommend going with the 1 hour charger its very inexpensive and comes with 4 AA batteries.
Some say they might be too powerful and if you pop your flash too many times in a row they will actually cause your flash to either over heat of just break/malfunction.
I will still need to spend more time with these batteries to see if the fear of over heating or breaking your flash is possible but I would say like anything else don't go crazy popping full power flashes just because you can... be a responsible flasher
and watch your heat!