On the paper, the Nickel-Zinc (NiZn) rechargeable battery is great for shoe mount flashes: Higher voltage and lower internal resistance than most NiMH rechargeable batteries, which translates into faster recycling times for flashes. How much faster? According to this document, it can be 50% faster!
Is that all good? It is definitely not if the voltage is high enough to fry the circuit of your flash. It certainly happened. Here are just a few examples:
The bottom line: if you need faster recycle times and are not afraid of frying your flashes, NiZn rechargeable batteries are certainly able to provide the thrill. Other than that, NiZn rechargeable batteries don’t seem to be good for anything else. You should be prepared to pay more for a NiZn-compatible charger and reduced cycle life.
For more discussions of using NiZn rechargeable batteries in flashes, read those Strobist discussions threads on Flickr.