I call it a spare tire. I know tire shops hate to work on tires with that stuff since it coats everything with a sticky goo. Personally I wouldn't use it since frequenty I do my own tire work, including dismounting/mounting and balancing
I have used several varieties and they seem to be all about the same. They are good for small punctures like nails. They will get you out of a jam, but they make a mess. Don't leave it in any longer than tkes to get to a tire store. They are all made from liquid latex and can unbalance a tire and make driving akward.
You can also buy a plug kit similar to what the tire shops use to fix a puncture... they usually come with a few sticky plugs, a reamer to roughen up the puncture and the plug insertion tool. You will want to keep a pair of pliers handy to pull the nail out. You'll also want one of those $20 12V compressors to refill the tire.
The one good thing about the fix aflat in a can stuff is that it will inflate the tire somewhat. I've used it before and it did work ok, however I also had a not so good experience with it...About 20 years ago I had a tire go flat on a lawn tractor. I tried the the fix a flat in can, but it still leaked a little. I brought it to the tire shop, and when the technician popped the tire off the rim he almost got knocked out from the odor... I got a bit of "freindly advise" from the shop owner for not letting them know I had used that stuff...
I would think the stuff they sell today is a bit more environmently freindly than the stuff I used back in the early 80s.
Like others have said, it is a good temp fix. If you leave a tire filled with that stuff, down the road when you do go to fix the tire, the latex has had so much time to harden that kind of welds itself to the tire/rime and the only way to get tire off is to cut it. I had a tire that was filled with fix-a-flat a year before. I could not break the bead with the tire machine. I tried breaking the bead using the tip of a backhoe. The backhoe (New Holland 650) did not break it either. We ended up using a sawz-all and grinder/wirewheel to clean up the rim.