l left mine on usb off and on for about 3 hours and the battery indicator went from about 50% up to about 90%. that was on a desktop machine, not a laptop though.
so at least while you're messing around with files with the unit connected you're taking on some fuel even if it's not the 2-hour rapid charge of an ac adaptor.
i do wish they'd've built the charging into the usb since you can get a charger with a usb connector anywhere for about $5 because every other cell phone uses that method. and the tiny little plug on their ac adaptor is hard to find and prone to structural problems due to its small size.