These are great little things: They produce a fair amount of smoke and the burning is so controlled that you can put them on a piece of cardboard and they won't burn through.
The only problems I can mention are that the smoke release rate isn't wonderful: Although they go for a while, you'll never have a view-obscuring cloud of smoke unless you let it off in a confined space - NOT recommendable because the smoke stinks and apparantly can stain clothes.
TL;DR: Easy and safe but buy several if you want lots of smoke.