+ Twice the power (or so it feels)
+ Great protection
+ Great choice if you have big hands (as yours truly)
- Potentially faulty mini USB port
- No front protection
- May be too bulky for people with small hands (but how would I know?)
In April, I managed to break my Speck Tough Skin case while demonstrating to a friend how (not) to take it off. Well, it appeared that while unbreakable on the rubber outside, the case plastic frame is very fragile. Or perhaps it is just my hands...
Anyhow, after that unfortunate incident, I decided to go all the way and order a battery/case combo from Mophie. Enter Mophie Juice Pack Air. In case you don't know, this is an external iPhone 3G battery pack that doubles as a hard case, protecting the back of your gadget (no screen protector included, which is a pity, a double combo would be nice). After an initial delay (apparently mophie.com could hardly serve the large number of customers, but they were very nice and responsive), I finally got my hand on a brand new Mophie Juice Pack Air (or MJPA for short). Initially, I was thrilled. The combo felt very Applish - nice white plastic, smooth edges... even the box looked like something out of Cupertino. Then, there was the power aspect. And I was thrilled again. Finally, I did not have to worry about recharging my iPhone after half-day usage. Even long calls did not drain my battery. I estimate the combined internal and external batteries give enough power for one day of extreme usage (long calls and/or games plus standard iPoding and Kindling) or two days of moderate usage (iPod and Kindle only, with some SMS-es and sporadic short phone calls). Which is great compared to the internal iPhone battery only.
As a hardcase, MJPA also works great. I am a harsh user and I have already dropped my MJPA-covered iPhone several times (about 0.5 m, on a bare floor). Not a scratch on it!
Additional points go to Mophie for making my iPhone more bulky. For my big hands, it is an almost perfect solution. Slippery as it is with its shiny polished plastic, it is big enough to be hard to drop. I can imagine, however, that some people with smaller hands (any female iPhone users reading that?) may find it cumbersome.
Now, here comes the caveat. Beware of the MJPA's mini USB port. Mine seemed fragile from the beginning. After less than a month, I tried to connect the cable and the port just snapped. It came loose and did not connect anymore. I contacted the support and a replacement was shipped immediately (even without my returning of the failed unit). The quality of Mophie's customer support is really outstanding, but the mini USB port is really something you should take special care of. I know I will. Hopefully, the second unit will live longer!

