In my experience, I've never had a plug leak. I wouldn't call myself an expert but I'll put it this way-- I have 5 sets of plug tools in my toolboxes. And one or two plugs. I've always tracked any leaks down to another nail/screw/glass/tack/bullet in the tire somewhere.

If you take a wheel into a shop w/ a whole can of Fix-A-Flat in it, don't use your real address when they log your car into the books!! -scott