I've ridden with Slime tubes for a period of time. I have never seen them actually stop a leak. As far as I can tell, all they do is make a mess that has to be cleaned up before you can patch properly. Even the tiniest holes made by thin bits of wire still leaked down.

I can't believe that my experience is typical or they wouldn't still be in business, but even when I bought my bike and it came with factory Slime filled tires (it was a cheap hybrid) the stuff didn't work at all.