I've had mainly good experience with the Nickel-Zinc batteries that I bought last year. My only concern is that the high voltage (1.8 V) would be too much for some devices designed for conventional 1.5 V ZnC or alkaline batteries, so in many cases I tried mixing them with NiMH batteries to get voltages such as 3 V from one of each.
People say you shouldn't do that. Why? They're in series, so the voltages just add up. But in practice, I've noticed that when they discharge, it's the NiZn battery first, and it shows alarmingly low voltages. The first time I thought it was possibly a defective battery, but it happened again today. The NiMH battery had a voltage of 1.2 V, almost normal, but the NiZn battery was showing about 0.6 V when I took it out of the device. By the time I put it in the charger, only a few minutes later, it had recovered to 1.235 V, and it charged almost normally to 1.835 V (in contrast to 1.866 V for the same battery in February). But clearly there's something wrong here.
Peter Jeremy came up with the expectation that it would be the other battery that would be reverse charged: although the battery is discharged, it continues to have current go through it. But that would apply to either battery, depending on which goes flat first. Clearly a weakness in my view of batteries as simply a source of voltage in series with a variable resistor. Another Don't Do That, Then .