These are great to get you out of a bind and I carry a kit. Back when I was in school-- and very poor, I used one at a gas station one night to pull a nail out and patch it right there-- much better than changing a tire. I drove on it for a couple years.
These are not as good as hot patch repairs (which seal the tire). The plug can decide to liberate itself-- and it most likely will do this when the tire is at it's hottest (at high speed). When this happens it causes a sudden loss of air (blowout). Thus, this is not as safe at high speeds as a hot patch. If you are going to use a plug follow the instructions very carefully. After installing-- I test the plug by overinflating my tire by a good 75% percent and I watch for the plug to blow out-- if it does not, I deinflate it to regular tire pressure and motor on. Watch the tire if you do this and make sure you don't blowout the tire by over-inflating it too much.
If you do a poor job with the plug, or the tire is damaged too bad-- the plug will blow out while you are overinflating-- this means your fix was probably not safe. Either work the hole some more and use a bigger plug or... go very slow until you get somewhere where you can get the tire fixed right (hot patch).
While I drove mine like this for 2 years, it would have been better to get the tire patched with a hot patch. Most tire retailers here do this for free if you bought the tire from them. I have relegated this to an emergency (temporary) fix only.