A while ago, I did some testing on Turnigy NiZn cells - unfortunately, I never got around to actually posting the results before now:
NiZn rechargeables have an advantage over NiMH rechargeables in that they maintain a significantly higher terminal voltage throughout their discharge, even when running at heavy loads (around 1.6V versus 1.2V) - this can be useful for devices that are poorly designed and don't work well on the lower voltage of NiMH cells. (Any device that can't work properly below 1.2V won't be able to use a significant amount of the capacity of even regular alkaline cells - with Alkalines, the voltage drops off as they discharge - at 1.2V, they still contain a significant amount of their original energy content)
The biggest problems with NiZn rechargeables are that they have a bad reputation for high self discharge and that they're quite fragile - many people have reported that they've only managed to get a handful of cycles out of their NiZn cells before they became unusable.
With this test, i've shown that NiZn cells are capable of performing quite well for a significant number of cycles. It appears though that when they're deeply discharged, that is when they sustain damage - and the damage occurs quite quickly. I performed a similar test with a new pair of NiZn cells and instead of using a 1.3V discharge cutoff, I discharged them down to 0.3V - the cells only lasted for about 80 - 100 cycles before dying in that test - still significantly better than has been reported, but a significant difference compared to the first test. I think that the key reason that the cells still lasted for a reasonable number of cycles in the latest test was because the load was removed as soon as the loaded voltage dropped to 0.3V - many devices will continue to slowly drain the cells in them even when the device has stopped working and has been turned "off" - I suspect that the slow constant drain causes more damage than a quick discharge down to a very low voltage.
In regard to the self discharge characteristics of NiZn cells, I've seen a wide variety of remaining capacities from cells that have been allowed to sit for a while - some cells seem to retain quite reasonable capacities for a number of months whereas others lose their charge over a relatively short period. Another downside to NiZn cells is that there are often large differences in the capacities between otherwise identical cells - combined with the highly variable self discharge rates and their propensity to be easily damaged when deeply discharged, it's understandable why lots of people have had problems with this type of chemistry.
In addition to devices which can't handle the lower voltage of NiMH cells, there are also some devices that can't handle the higher voltage from NiZn cells - I've seen reports of devices dying due to this higher voltage. Camera flashes can cycle faster with the higher voltage, but if continued at a high rate, it can end up over heating and killing the flash altogether!
Overall, I think that if the variable capacities and self discharge rates could be improved with better quality control, there could still be a future for NiZn cells. If they could be improved so that they can better handle deep discharges, then that would also help significantly. Even with these improvements (if they ever come) NiZn cells will still only ever be useful in niche roles - Eneloops (and other low self discharge NiMH cells) are still more than good enough for the vast majority of applications.