So that I always know where my bike pump is, I keep it in the below floor storage in the van. I always carry a plug kit with me. It's been years since I've needed it, but if you can quickly find the leak with the tire still on the car, (a nearby puddle to drive through makes this very easy, if you're lucky enough to have a puddle nearby) it takes less time to plug a hole and pump the tire back to regular pressure than it does to change the tire (particularly since you have to change the tire twice, unless you have a full-size spare. I've never had a plug fail over the remaining life of the tire (10's of thousands more miles), though the NHTSA doesn't consider plugs to be permanent fixes.
However, plugs are 100 times better to use than those fix-a-flat spray cans.
And, I would never plug a sidewall. Allegedly, they can lead to a blowout. I can understand why, since the sidewall is under different types of strains/stresses. The original hole acts as a weak point allowing the tire to suddenly tear open - not an issue on the tread. Also, the plug kits I use have some sort of glue with them - you rub the glue over the plug before inserting the plug.