A USB port by design will provide a minimum of 5v and .5 amps, which will work quite well for vaping. However most atomizers will draw more current (amps) than .5. You may get a message on you computer that the device is doing this, and it may shut it off. What most people use is a simple USB wall or car adapter to power their USB passthroughs. And yes, the Puresmoker USB passthru is the only commercial passthroughs that I am aware of, it is undergoing redesign right now however and is in preorder.
The way the cheap chinese passthroughs work is to use a rechargeable battery that is actually just running 3.7v and it's connected through the USB so that it's constantly recharging. You're technically not actually running your ecig directly off the computer.
The only manufacturer I know that uses the battery inline is Joye for the 302/510/306 atomizers, most others are simply tossing a circuit board inside that limits the voltage to around 3.7 volts. There is a good reason for this, to extend the life span of the micro switches they use. They also tend to all use very thin gauge wire, and have a reputation for very short life spans.
I use a home made USB passthrough, which provides me with the voltage I like. It surprises me that more manufacturers haven't come out with their own designed passthrough's.
