I'll be restating some things that have already been said, but consider it verification
through multiple sources.
I've seen it said that they can't be used with the AC adapter and now that they have been.
You should be able to use a PT with any USB adapter that provides at least 1000mA(1A) to the port.
Obviously the advertised convenience of them is that they can be used with your pc but I've also seen a post or two saying it's a bad idea and may fry your usb port or even motherboard.
Highly unlikely. Most modern motherboards and operating systems are smart enough to cut power to a USB port drawing too much. If that doesn't happen, even in 95% of cases the worst you'll do is blow out that USB header usually causing you to use 1-2 USB ports. Only in a truly worst case scenario will you destroy your motherboard. However, this can be the case with any single
device you plug into a USB port that draws power, and is typically not a problem. If you're concerned about it, consider an external powered USB hub.
I also haven't bought a car adapter for it yet but I do have a stereo in my truck with a usb port in it. I was skeptical about using it in that for fear of blowing a fuse, but I just couldn't resist, and last night I used it on that usb port on 2 different 20 minute drives with no issues at all.
I am surprised to hear this one, I wouldn't have thought that a USB port on a head unit would provide full power, but it's certainly a good thing. There still might be concerns as the head unit's USB port may not be designed to provide full power
through it. I would still suggest using a car adapter if it's feasible, because like you said blowing a fuse wouldn't be fun.