Nice battery, but at least 4x overpriced. Besides, I am very sceptical of their claim of it fully recharging an iPhone in 15 minutes (skip to the bottom line if you're not interested in technology )
Apple's standard A/C charger takes over 2 hours to fully charge an iPhone 4 (empty to full). If this battery were to do it in 15 minutes, an iPhone would have to draw 8x the current from it, which it will never do because it would be highly unhealthy to the battery it is charging. Li-ion batteries are virtually never charged with a current of more than 1C, which refers to the current that would fully discharge it in one hour. Usually the limit is 0.5C-1C, often 0.8C and less. Fully charging consists of a constant current phase, in which this limit is applied, and a subsequent constant voltage phase, in which the charge current becomes smaller and smaller until it has dropped to a few percent of the initial current. So you might see the battery going from 0 to 90% in a bit more than an hour, then it might take another hour or so to go to 100%. Don't follow those rules and the numbers of times the battery can be discharged and recharged goes down drastically. Since Apple doesn't want to replace more batteries under warranty than they have to, they naturally design the iPhone to limit the charge current to a sensible value such that this doesn't usually happen and you can get 2-3 years of usage out of it before it needs to be refurbished or replaced. Also, if you were to initially charge at 2C or 3C, that would not shorten the lengthy constant voltage part of the charge. You could only quick charge to about 90%, but not speed up the final 10%.
Bottom line: Anyone who tells you their charger will fully charge your phone battery in under an hour is telling you nonsense.
EDIT: I read the blurb on the maker's site in Japanese. No, it won't fully charge your iPhone in 15 minutes and they don't actually make that claim, if you read it carefully. The mobile USB battery itself will pick up 1440 mAh of charge (the capacity of an iPhone battery) from a wall socket charger in 15 minutes, which it can later pass on to an iPhone ("iPhoneの満充電一回分の容量(約1440mAh)を、わずか15分でコンセントからモバイルバッテリーにチャージできます。フルチャージ(8000mAh）は約94分で完了します。"). Of course all the English articles copy the mistake from the Akihabaranews writeup