The Mophie feels robust in hand and ready to travel with you anywhere. It takes around 9/10 hrs to charge fully with the Lumia 920 original charger at 1A and 7-8 with a Nokia 1.2A charger.
I have charged my Lumia 920 twice from around 20% remaining and it still had more for another charge. It charges at around the same rate as the wall charger so no slow charges from this as others do.
If you want something that fits in your bag, go anywhere and keep your power hungry devices going then this is the answer. The only slight down side is that the connector covers could do with being slightly more of a snug fit in case of exposure to water.
If you need more capacity then try the Mophie XL (1200 mAh) which will charge two devices simultaneously.
Give it a go, you wont regret it!