I ordered a few sets of PowerGenix NiZn (Zinc based) rechargeables to give them a go. Their claim is 1.6v (vs 1.2 for NiMh) and highdrain. I've been VERY happy with the performance of Eneloops. I get 300-700 pops out of a charge, they don't self drain, recycle doesn't slow down after a few pops like Alkalines, They recycle faster than Alkalines(a lot faster), they don't heat up like Lithiums.
Still, always looking for something better. A few devices can't live with 1.2 volts, like PocketWizzards TT5's.
No field testing with the NiZn's yet, but I just did a stopwatch test the blew me away.
580ex-II on manual / full power. I press the pilot button, start the watch, wait for the light to glow red stop the watch. Eneloops, 2.79 seconds, and holds pretty steady, even after a dozen pops in a row. The PowerGenix's, 1.64 seconds, and hold steady, even after a dozen pops in a row(no rest). The NiZn's came out a bit warmer, but not HOT like Lithium's.
WARNING I ran a dozen pops again, to confirm my results before posting. My 580 is DEAD. At first it would power on, but not charge. Now, after a cooldown, it won't even power on. Smells burnt too. So, I've killed it. The batteries are better, but, more than the flash can handle.