I bought this USB charging unit because I have a lousy sense of direction. I get lost in buildings and in cities, even ones I'm familiar with. My iPhone's GPS map is a godsend. I use it when I'm driving, walking, and taking public transportation.
When I was in Tokyo in June, I brought along a small Android phone installed with a local data SIM card. I used the phone as a wireless hotspot for my iPhone, and appreciated having access to the online map to guide me from my hotel to the subway station, and to attractions like the Tsukiji fish market and Kabuki-Cho. It was also nice to call home using Skype, and to post Instagram photos. The only problem was that the batteries on the Android phone and iPhone drained after a few hours, forcing me to ask people, chikatetsu wa doko desu ka? in badly-accented Japanese, to find the nearest subway station.
On my next overseas trip a couple of months later, I brought along a 6000 mAh Mophie Juice Pack Powerstation Duo. It's a beefy battery pack that has two USB charging ports. I kept it in my daypack and my family and I used it to charge our phones while we were walking around London and Paris. It was like having a portable wall outlet, because it never came close to running out of juice, despite nearly constant use. It's bulky — about as wide and tall as an iPhone 4 and nearly three times as thick (see photo below). I wouldn't carry it in my pants pocket, but it isn't a problem in a jacket pocket or bag.
If you want all-day smartphone Internet connectivity, the Mophie Powerstation Duo is what you need.