I have used plugs 2-3 times and have never had one fail. However, if I were going on a long trip and needed to travel interstate speeds where tire heat really builds up, I'd get a new tire. Cheap insurance and piece of mind. My rear tire is plugged right now and has been that way for the past 1000 miles.

 

I took a nail to a 200 mile front tire. I replaced it with a new one. Better to be safe than sorry.