To echo
@Lain and also having used both Metas and Vanilla VGCs regularly at Costco (just yesterday actually), it's incredibly easy. I actually don't even register my cards (either variety) and the Costco here (MD) doesn't prompt me for a PIN either (if it does, I usually just press enter and it will run as credit presumably). As
@Lain mentioned, they auto-drain, which is lovely, since it's one place I can easily get rid of all the $0.XX leftovers from buying MOs. I think I finished off a stack of 5 $0.XX cards once (at the pharmacy or else others in the long normal checkout lines might not be happy with me lol.
And if you use them in store, the VGCs are wonderful, no activation required once purchased.