My concern with the USB thing hasn't been cooking a battery, but cooking my computer. I read here somewhere about a USB plug that somehow screwed up someone's computer. (Might have been a passthrough, it's a vague memory.) Anyway, after that I quit putting any of my battery stuff in my USB port and only use an AC adapter.
I use the USB>AC plug that's come with some of my eGo kits and they charge really fast. I've not timed it, but it's not six hours. I usually have three eGo's on chargers and a couple already charged at any given time and just rotate through them as one dies.
Like I said above, I like to have plenty of spares.