My personal experience hasn't been good. I've had one puncture so far and the slime did nothing but make everything, well, slimy. I took a staple in the rear tire (the kind that you fire out of a hand-held Arrow gun, not like a plywood staple or something. As far as I can tell, the slime didn't even slow the leak down. It just made it so I had to clean up a bunch of green crap before I could patch the tube.

I'm running Kendas at about 75 PSI. They came from the LBS with Slime in them. I noticed the hiss immediately upon picking up the staple. I was 100 yards from work. Stopped, pulled out the staple, turned the tire leaky side down. Went inside, came out 5 minutes later to a completely flat tire.

When I switched to winter tires a couple of weeks ago, I put in fresh tubes without the Slime. I checked over the tube pretty carefully, and there were no other holes that I noticed, and I assume I'd see them because they'd have a green slimy marker. So out of one incident consisting of two 1/16" long by 0" thick slices, the stuff that claims to instantly patch holes up to 1/8" in diameter (that's a pretty big hole!) scores 0%.

Just my experience, and I'm a first-year commuter, so YMMV.