WPxbox have posted this review of the very affordable Brando Power Jacket for the Nokia Lumia 920.
The 2,200 mAh battery pack costs only $37 and provides protection and doubles battery life, all in a very convenient package.
The case adds another 85 g to your Lumia and build quality is reasonably good. The main disadvantages is the added bulk, and it may block the microphone slightly, but it otherwise seems like a steal.
Read the full review at WPXbox here.