Incase Snap Battery Case for the iPhone 4 is a similar concept to the Mophie Juice Pack for the iPhone 4 in that it offers an extended battery inside a hard case for protection on the iPhone 4. Instead of a full wrap around case on the Mophie Juice Pack Air, which is an excellent case, Incase designed to use a snap-on design that leaves the two sides of the iPhone 4 exposed. This is probably a design decision to make the volume buttons easier to access, which in my experience with the Mophie case made it hard to access with the Juice Pack.
Also, rather than relying on a micro USB port on the case to charge and sync the phone and battery pack, Incase has gone ahead and utilized a pass-through dock connector to another dock connector so you can use your original iPhone cable. This really depends on the preference of the user as a pass-through to micro USB solution on the Mophie may make it easier for those who have a lot of micro USB to USB cables laying around, but for those who want to use an official USB to iPhone dock connector cable, then the Incase solution will work out better.
The company says the case features a 900 mAh battery, which is smaller than the Mophie Juice Pack Air, but will offer a slimmer case:
Precision engineered to be the slimmest battery case for iPhone 4, the Snap Battery Case features an integrated 900 mAh battery and durable hardshell construction for increased battery life and reliable protection.
This battery will extend the battery life of the iPhone by up to 40% to give you a bit more boost through your usage. The case retails for $60 via Incase.