I bought a Mophie Juice Pack Air from the Best Buy Express in MSP one trip in December when I forgot my iPhone charger. I really like it because it uses a MicroUSB to charge which meant I was able to borrow chargers from my friends with Android phones and it also has no problem charging from the USB in my car stereo even though my phone will not.
We’re not even in April yet, I’ve maybe taken the thing on 5 trips and the Micro USB had broken off and is just rattling around in the case. Apparently this is a known issue from previous versions as well because a simple google search turned up this video from the iPhone 3GS:
(NOTE: don’t actually open it because you can’t fix it!)
I like the form factor of a wrap around case and I like the Micro USB charging because that gives me 2 options to charge and I usually have a Micro USB cord for other reasons anyway, but sounds like this case has issues.