A glass chip gashed the front tire of my bike a while ago, but the slit didn’t cut the Kevlar belt underneath and I let it slide. The pre-ride check before our 50-mile day trip to Old Rhinebeck Aerodrome showed that things had gotten worse (the tire liner was peeking out through the belt), so I replaced the tire before we set out.
This pic shows that the tire liner was doing its job, although it was slightly abraded and had picked up some road grit. The tube had a barely visible mark.
I generally use fluorescent green Slime tire liners, but this one is a competing brand I picked up a while ago. Not much to choose between the two, although I think Slime liners have a better edge taper and tend to be more flexible.
Notice the other nicks and gashes in the tire tread? We run Schwalbe Marathons on the rear and Primo Comets on the front, both have Kevlar belts. Flats are not a problem any more, even with plenty of sharp road debris; I replace the tires every two years or so when the tread wears smooth or a major gash worries even me. My rule of thumb: when I can see the liner, it’s time to replace the tire.
There are riders who argue for very lightweight tires on the basis of performance: better acceleration and lower rolling resistance. I’m willing to trade all that off against not having to dismount a tire by the side of the road…
[Update: Plenty more posts on this general subject, with graphic illustrations of tire damage. Search for liner and you’ll find ’em.]
The Smell of Molten Projects in the Morning 
Cool! I recently put on Marathon’s too because I kept getting flats. I mean multiple flats per day. What a PAIN! No more flats after the Marthons. I can see the point of lighter tires but my main goal is exercise not speed. I don’t use the slime anymore either since I also put in ‘thorn resistant’ tubes. Yes it is heavier and I don’t go quite as fast but my two hour ride is only a few minutes longer. I like the piece of mind Marathons give. Now I laugh at the “Thorn God’s”. They can’t touch me anymore.
A friend refers to the God of Flatulence, who’s in charge of things like flat tires: he always wins in the end!