500amp? 
fuuuck! I'm assuming you mean 500 ma (milliamps).
I don't know how that compares to a pc usb but there is another possible reason to use a wall charger - rumour has it that it can fuck up your computer. When I was charging off my laptop I got quite a few blue screen crashes. This could be a complete coincidence - the lappy is very ancient and I'm always mildly surprised it still fires up at all, but I have seen a lot of people recommend that you don't charge off a computer if possible (this might only apply to laptops, I don't know - I might see if I can formulate a coherent question for the geeky stuff topic on this)
I've done a lot of reading around since I first read the ecita advice. Lots of people dispute it. What I have gathered is:
- You won't go wrong if you limit your wall plug to 500ma but as long as you are using the correct ecig charger there should be no problem using a higher amp plug. If you are not sure and are at all worried, use a 500ma wall plug.
2) never ever ever ever ever use a cig-alike charger for an ego style battery. All the exploding ecig stories I have seen have been this setup. Cig-alikes have internal circuitry which steps down the current so the chargers they come with are just basically adapters. Egos don't have this circuitry in the battery, they rely on the charger to step down the current. The charger should have an OUTPUT of 500ma and 4.2V
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MVPs have internal protective circuitry like cig-alikes, which is why they come with what is basically a USB lead for charging. I charge mine on a 1.5A wall plug and have never had a problem, although being a nervous sort I never leave it unattended.
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The very worst set up you could possibly have is apple charger (higher amps than standard) + cig-alike charger + ego battery. That was the setup in the bar explosion.
Further reading here and here.
This is all clear as mud isn't it? If there is one area we need better regulation it is this. All it needs is clearer labelling.