Well, it depends on the situation. Like I said, they tend to self discharge a sizable portion of their power, (discharging as much as 10% in the first day, and roughly 5% every 10 days thereafter) making them significantly less efficient than a typical lead-acid battery. But, for certain cases, the nearly indestructible nature of the battery would make it a perfect candidate for places like an unattended cabin where the possibility of complete discharge could occur. (something that would kill a lead-acid battery in short order) The main advantage would be, it would likely be the last battery bank you'd ever have to buy, as they usually last a lifetime. (or longer) About the only thing you have to do is keep the electrolyte topped up and change the electrolyte ever 12~15 years. Being that they accept deep discharging far superiorly than a lead-acid, you can get away with a smaller bank of batteries. Lead-acid shouldn't be discharged below 75~80% capacity, as it will shorten it's life significantly if you do. A nickel-iron battery can be safely pulled down to 50% (and even lower) without harmful effects.

Comparison:
1000 amp/hour lead-acid bank, 75% discharge = 250Ahr or 25 amps for 10 hours.
500 amp/hour nickel-iron bank, 50% discharged = 250Ahr or 25 amps for 10 hours.
Half the capacity, same amount of available power. (technically more)

OK, another factor. Nickel-iron batteries have a lower cell voltage than lead-acid, (1.2v vs. 2.1v per cell) so more batteries will be required in a string to achieve the voltage needed. With their relatively higher price, it can make for a pricey battery bank, but when you consider that a lead-acid bank will get changed out several time in your lifetime, in the end, it's cheaper. (if you can absorb the high upfront cost)

So, if you can live with the upfront cost, (unless you're lucky enough to come across an old set for free) the lower efficiency, and a slightly different maintenance schedule, (more regular watering, but less equalizing charges) they are far superior to lead-acid. It's something each individual has to weigh for themselves.