The original "Juice Pack" from Mophie was one of the first cases for the iPhone that included a built-in battery, but it wasn't really much of a case. It went halfway up the back of the phone, leaving a lump not just on the back, but along the bottom as well. All the hassle of a case—with only half the protection.
The new model, the Juice Pack Air, keeps the 1200-mAh battery but slips it inside a full plastic case. The chin is less lenoesque, as well. I'd go so far as to say that that in some circumstances it makes the iPhone more comfortable to hold.
And I'm someone who almost unilaterally hates iPhone cases. The iPhone is a really nice size, but its width is borderline too big. Without a case it slips into any pocket, including the front of my sexy jeans that I'm now too fat to fit into. (Thanks, Oregon!)
A switch on the bottom of the Juice Pack Air turns its internal battery on and off. With it on, the iPhone will think it's constantly being charged up, using up the Air's battery before it uses its own. You'd think you'd want to leave it on all the time then, except that even Mophie acknowledges that there's a slight hit to the battery life compared to waiting until the iPhone is drained and recharging from the Juice Pack. Except for the most fretful, I expect you'd be happier just leaving the Juice Pack on and dealing with a little bit of loss.
I charged both the iPhone 3GS and the Juice Pack Air fully and got a full three days of use, including a few minutes of game playing, an hour of audio recording, lots of random web browsing and tweeting, and about two hours of phone calls before I (nearly) ran dry. Bear in mind, though, that the iPhone itself had battery for the better part of the first two days. (Complaints about the iPhone battery life tend to be, in my limited experience, complaints from people who can't discern why playing a 3D game might take more power than, say, listening to music.)
As a case, the Juice Pack Air is nothing special. It adds most of its thickness in the back, which actually makes the iPhone feel like an old iPaq or something. It still fits in a front pocket, but it's a bigger lump. Totally manageable, though. Its looks are nothing special, basic black plastic with no texture. A bit cheap feeling, actually, but solid. There are four little LEDs on the back that show the Juice Pack's battery level when you push a button, a la the MacBook. (I wish they were smaller like the MacBook, but it's not a big deal.)
One minor kink: the Juice Pack Air plugs into the iPhone's Dock Connector at the bottom, but doesn't have a normal pass-through, but instead uses a microUSB port. For most people that's not a big deal at all—it still charges both the phone and the Juice Pack Air, as well as allows syncing—but if you use any accessories that need to stay plugged into the Dock Connector, you'll have to remove the iPhone from the case.
The Juice Pack Air goes for about $75 on the street—not horrible, but there's definitely a premium going on for getting an all-in-one unit. If you just want a spare battery, you can grab a cheapo 1000-mAh battery for ten bucks. (Or a similar 2400-mAh case for $26 that doesn't look to protect quite fully.)
I guess what I'm saying is that the Juice Pack Air is probably too expensive—but if you've got your heart set on it, I can attest that it's the first case that I've considered keeping on my iPhone in a long time.