When I traveled to Alaska this summer I went looking for a travel USB charger to take with me. My brother and I roomed together most of the trip and between the two of us we had two iPhones, a 3rd Generation iPad, an Android Tablet (not mine), two digital cameras, a battery backup, a Kingston Wi-Drive and a bluetooth keyboard. Each of these items charged via USB. In our stateroom on the cruise ship we had a grand total of one wall outlet available for charging up all these gadgets and any other power needs.
My solution for charging all our gear quickly and efficiently was the Kanex DoubleUp. When researching USB chargers I had a couple of requirements. First, I wanted something made by a reputable company and certified so I didn't have to worry about it zapping my gadgets or causing electrical problems. Second, I wanted something fairly small size as space was at a premium. Third, it had to be able to charge multiple devices as I did not want to carry around multiple power adapters. Lastly, I wanted something that could charge two full size iPads simultaneously since my family travels with three full size iPads and now apparently one Android tablet. The DoubleUp fit the bill.
The DoubleUp retails for $34.00 ($29 on Amazon) and is available in black or white. Perhaps the most unique feature about the DoubleUp is it has two full-power 2.1A (10 watt) USB charging ports meaning it can charge two full-size iPads at full speed simultaneously. It will also fast-charge an iPhone or other smartphone. While "fast charging" of iPhones may not be documented, Apple, I and others have noticed the iPhone will accept more than the 5w charge officially supported and thus charges faster with the DoubleUp. A couple of nights my iPhone had less than a 20% charge remaining and I plugged it in to the DoubleUp while I was up at dinner (maybe an hour to an hour and a half) and returned to find the phone completely recharged.
Another convenient features of the DoubleUp is that it has LED charging lights that will shine amber when a device is charging, green when it nears a full charge (90% for an iPhone or 98% for an iPad) and will turn off completely after your device is charged. This allows you to tell at a glance whether your device is charged and ready to go without having to walk over and turn on the screen. In my experience, the lights were subdued enough I didn't find them disrupting when trying to sleep at night in a small hotel room or boat cabin.
The DoubleUp is about 3 3/8" long, 2 1/2" wide and 1 1/8" thick and is a good bit larger than either an iPhone or iPad wall charger, but probably doesn't take up as much space as traveling with both an iPhone or iPad charger. The prongs fold away for more compact storage. That being said, I wish the DoubleUp was smaller as it's a device that's ideal for travel.
The only complaint I have about the DoubleUp, other than the size, is it seemed to be bottom heavy. This meant it would easily pull or fall out of outlets if it was not hung vertical with the USB ports facing downward. Unfortunately, many cases I had to plug the DoubleUp in upside-down or sideways due to the arrangements of plugs and on multiple occasions had to keep fidgeting with it to keep it to stay in the plug. One night while on vacation DoubleUp was positioned upside-down and pulled out of the outlet during the night and I awoke to a dead iPhone. This was not fun, but on the plus side by the time I took my morning shower and was ready to go my iPhone was about 90% charged thanks to fast charging.
Until recently, I haven't seen a product similar to the DoubleUp, although just this week Jeff at iPhoneJD reviewed the Kingston Absolute Power 4.2 which appears to serve a similar function. All things considered, I'm very happy with the DoubleUp. It saves me space and hassle by avoiding traveling with multiple accessories and it's earned a permanent place in my travel bag.