The APC UPB10 power pack worked great throughout my trip around Europe. Between the iPhone battery and this external battery, I could pretty much watch movies all through 12-hour flights and 8-hour train rides. Also as a generic USB power supply, it can charge bluetooth headsets and other devices.
Other reviews have mentioned the need to press the button every 15 minutes to keep the power going, and that did cause me to hesitate a bit before ordering this product. However, I've found this to be the case only when the device I'm powering is already fully charged and no longer drawing much external power. In other words, it will fully charge a phone from empty unattended even if it takes 3 hours, but it will not continually power a fully-charged iPod because the power draw is too low, even when playing music or videos. Yes, maybe they could have had a switch, or maybe the threshold could be lower, but I felt it to be a non-issue once I understood what it was doing. I just use it as a charger and not a continuous power source.
This is a fantastic product, I highly recommend it.