We have been sent a Lumsing Power Bank 10400 mAh portable USB charger to give a whirl, and are happy to report after a week of use the device is a definite buy.
We tested the device by seeing how long our Nokia Lumia 920 would last away from a power socket, relying only on the Power Bank to keep the device powered.
Starting with a fully charged Nokia Lumia 920, the device managed to keep my handset going for 4 full days, which I think is quite an achievement for a WP8.1 handset. The charging lights also did a good job of keeping me informed of the current status of the device, though the last 25% seemed to go much faster than the first.
The device has some nifty tricks, including being able to charge two devices at once, and of shutting down automatically after 5 minutes after fully charging your phone. Of note though is that this means it will not keep your phone at 100% overnight.
In terms of design, the ability to use the same wire to charge the battery as to charge your phone is quite nifty.
Recharging the battery to full from a laptop takes around 4 hours, which is relatively fast.