Redflare brings up a good point on fixaflat products. Some manufacture's use flammable gas for the propellant, read the label to see if it will make your tires go boom.

I have used plugs for years and have never had a problem. Finally broke down and purchased a safety seal plug kit which i highly recommend.

http://www.safetyseal.com/store/autokits.htm

These are more expensive than the discount store plug kits but it is very well made.