Buffalo has just announced a new backup battery called the Voltissimo that makes the bold claim of being able to fully charge an iPhone in just 15 minutes. While there's no technical reason it shouldn't be able to deliver on that claim, it's not the best idea for your phone in the long run.
While being able to resurrect a dead iPhone back to full capacity in just a quarter of an hour sounds like a dream come true, its battery isn't quite designed for such rigours. As batteries charge they get hot, and too much heat can actually damage and diminish the lifespan of the chemicals inside. You can buy rechargeable batteries that are specifically designed to be safely charged very quickly, but there's a reason the iPhone's included charger takes a couple of hours to do the same.
If you're the type who gets a new phone every year it might not be that big a deal, but, as a general rule, unless you're in a dire emergency and need a full battery, it's best to avoid those rapid charging kiosks and devices like this. After all, patience is a virtue.