i have had slime in my mountain bike tires for the last three years or so (maybe more) no flats.
of course i also have that plastic thing that goes between the tire and tube and heavy duty tubes too..
most of my riding (in 2000) was to and from work. decided to ride my bike, and kept at it from feb to november at least four/ five times a week. 15 nmiles each way.
i used to get lotsa flats at first, but the slime did the trick. the tubes and plastic thing werent as effective as when i used the slime. i still keep using all three because if one preventative measure is good, then three must be better.
now that i play rugby i dont need to kill myself riding to work. i just do it on the rugby field.