I have plugged plenty of tyres all with perfect success. The only time I have had a bad plug was when the AA man did it.
Generally, so long as the puncture is in the treaded part of the tyre, you should be good to go. The arguable question after this is if you should then proceed to get it repaired properly. There is always a risk that the carcass of the tyre has been damaged. You can't see this from the outside of the tyre. Also if it is plugged from the inside out, the air pressure ensures the plug keeps the seal.
Saying that, I have never had a proper repair done after a puncture and to date never had another issue with the plug I have put in.