I recently spend the extra $$ on the good plugs and already 2 of them are toast. I don't know why that would be, but I guess replacing copper plugs a bit more often isn't that bad.
I have a buddy that had the same thing happen to him, it caused some weird knocking in his engine. I think they were the fancy iridium plugs, I had the stealership replace mine a while back. I assume that they put the cheapos in. I guess it's pretty much a wash since although you should replace the plugs more often you pay less for the plugs.