"Completely worry free*"
* Unless you are worried by high-pitched squeals. It sets my son's teeth on edge, but he adjusted pretty quickly -- he gets to play Rage of Baha Men (or whatever) longer between trips to the outlet, so he's willing to make the sacrifice.
As for me, I'm happy because this charger, though small, does what the Energizer versions don't: pass-thru charging. I can plug the power pack into a USB port, and plug my phone into the power pack, and the power pack remains charged while the phone charges. That lets me leave the whole assembly on my nightstand charging, then take the phone and backup with me into the urban jungle for all-day power. (Maybe... I haven't actually tried this scenario, but it *should* work.)
There's one very interesting side-effect of this capability. It means that the power pack doesn't "pay attention" to which way the juice is flowing. If something wants power, it gives it. If something gives power, it takes it. Even another charger. Even...
... well, the first thing my son did was plug the unit INTO ITSELF. Sure enough, the light came on as the unit charged itself from... itself. Visions of perpetual motion danced in his head until I brought him down to thermodynamic reality. I felt like I'd told a 3-year-old the awful truth about the Man in Red. 
Cr@p-free no more as of 5/26/2010!
(Previous B-to-the-OC: 11/17/2006)
i can haz
quality post? zomg!
