Funny this question should come up. I've used slime tubes on and off for over a year and had not noticed any difference in the frequency of flats (quite a lot, with all sorts of tires in my hostile urban milieu).
Then just the day before yesterday I was on the final leg of an eighteen mile errand around the West Side on my mountain bike, 26 x 2.0 Serfa Drifter up front, 26 x 2.0 Schwalbe Marathon Plus Tour on back (I have had flats on both these tires). Forget what kind of tube is in back, an ordinary one, but in front I put in an old Bontrager (??) slime tube I found in the garage.
Riding home I look down and see a little geyser of some kind of fluid jetting out of the front wheel, just two rotations of the wheel then none. Yep, a puncture flat stopped in its tracks, only lost a minimal amount of pressure if any too.
So is slime worth it? I dunno. Slime tubes are more expensive and less portable and for me only worked that I knew about on that one puncture. Prob'ly wont go out of my way to look for them.
Mike