This is a great idea, I already know how useful this idea is because I already bought a power box for Androids etc from Maplins, if I'm doing protest coverage I can keep on Bambuser or filming for longer than you would normally on a single phone battery charge, take your phone anywhere far from a charger and you'll end up needing this.
That Maplins box was £25 and this is very nearly free.
The maplins box gives you about 2 or 3 times the normal duration of the phone battery (will depend on model and power usage etc), it will be interesting to see how this (almost free) gadget will compare. Luckily I'm on EE and can take advantage of this to get another one, I like the idea that if you're near a EE store and on low charge you can actually just swap for another power bar that's already charged up. You don't own it, which is a bit bizarre, but I'm a hoarder and will probably just pay the £5 to own it if I ever leave EE (which I might as Three offer unlimited 3G and 4G).
If you're on EE just get it, it'll save you from a lot of dead battery situations. Just remember to also charge this power bar too, unless you're always near one of their stores, which in my small town I won't be!
They won't easily get all these chargers back, I'm confident in keeping mine!
If you're on EE, text "POWER" to 365 and then you will be sent a code which can be used to claim one in an EE store.