<< Has anyone used that Duster stuff from Walmart/Kmart? >>
That stuff is by no mean air. It is liquified fluorocarbon. The cheap kind is flammable and is Difluoroethane(Dymel 152) and the expensive non flammable stuff is 1,1,1,2 tetrafluoroethane(R-134a). The latter is exact same thing as the refrigerant in modern automotive A/C and refrigerators. At one point "canned duster" was based on pure Freon 12.
I have about 12 cans that I stocked up when it was $2.99/2 on sale. They later had it on sale for $1.99/2 once. I wish I stocked up on more. Its usually quite expensive, something like $4 each, but first paragraph pretty much sums up why it isn't so cheap eh?
For big clean up, I take the computer outside and blow it with my yard blower. It beats vacuum since the nozzle doesn't have to make contact with anything to blow out dust. You'd be amazed how much dust woosh out when you do this.