Slime Self-Sealing Smart Tube 29er will instantly seal most punctures in the tread up to 1/8?(3mm). SLiME is Not Guaranteed to seal sidewall punctures, pinch flats, glass cuts or punctures over 1/8?(3mm).
Strengths: I've been riding a Surly Karate Monkey with these for about six months, mostly on canal banks and dirt roads. Here in the San Joaquin valley in central California we have these horrible thorns called goatheads (Tribulus terrestris). Yesterday I pulled over 50 of these out of my tires at one point after accidentally riding through a patch. I had four little pinpoints of bright green as I pulled out the thorns. That would have been four punctures at once, without Slime Smart Tubes. I also have Smart Tubes in my 700C hybrid, and will put them in the 2x9 29er hardtail I just ordered.
Weaknesses: You have to be carefull and have the valve stem at 10:00 or 2:00 to pump up the tire, or to let air out. Once I gummed up the valve with slime when I was letting all the air out of the tube. Removing the valve, cleaning it off, and screwing it back in solved the problem, which has not recurred now that I am more carefull when airing up the tires.