Its a good backup, but it won't fully charge your battery. The 720's battery has a capacity of 2000 mAh, that supplies 1720. Shop around, its not the cheapest, but you know its probably good quality.
i would say go for a bigger capacity (so that your phone last longer and keeps the battery healthy)
just make sure not to purchase one that has high capacity with a too good to be true price.
Even those with genuine stickers can be faked (one brand that has been victim is Yoobao)
Faked power banks mean problem.