These work surprisingly well, out of 3 uses (on 3 different bikes - 2 road, one mountain), all have been good repairs with no significant air loss afterwards. I figured they'd be good for a temporary repair and that I've have to replace the tube later, but I found that the repair was permanent and I'm still riding on the repaired tubes.
For years I rode my Gary Fisher Wahoo everywhere I went. I'm convinced there is no such thing as a convenient time to get a flat tire. With these patches in my pack, I am able to get back on the road, including pumping and everything, within literally 5 minutes of the tire going flat. For the price and convenience, it can't be beat. If applied correctly, it will last virtually forever as a permanent fix. I've only ever had one patch fail.