This review is from: New Trent Astro case IMP180 with Extended Rechargeable Battery Juice Case Air for IPHONE 4 4G (Electronics)
I now always have this battery case with me, and it's a relief knowing I don't have to be so careful about how much I use my iphone in the day. When my phone's battery is almost completely empty, this charger gets it up to about 60% or 70%.
It's got to be the smallest battery case I've seen, looks and fits well.
Only problem I had at first was trying to take off the top of it to get the iphone out (at one point I did worry that I'd never get my phone back!), but once you get the hang of it it's fine.
I even bought another one for my Dad, who's always complaining about the battery on his phone, and he seems quite pleased with it, which really is saying something!