My handheld GPS is always in the most need of Power. This wouldn't work for that though, so I would say my PDA, when traveling, is always in need of additional power.
A device like this one-wow-when I am have a work day with multiple meetings at multiple locations, my Volvo and I sync up with my Blackjack w/ Bluetooth and I burn throught batts like a wild woman.
I would love a device like this because I am sick of having my calls start dropping just after lunch. While this is a well known weakness of Blackjack batts, it is maddening!
My AT&T Tilt can go a day or two without a charge, and while I typically charge it every night, carrying around this power pack could save some close calls I've had... no pun intended.