Snake Oil with a nice smoke show to let you know how well it works.
Seriousely.........
A.) It only smokes when you put straight into the engine, through a vacum line into the intake. This is because this method is the most effective/rapid at removing internal deposits.
B.) The smoke is the engine burning off carbon deposits loosened by the SeaFoam when used through the intake. It will also only smoke on a dirty engine. The Neon I mentioned above stopped smoking after the 3rd can, and it ran like new, as opposed to running rough, and having an intermittent misfire previously. Yes, it had a mechanical problem (exhaust leak) that had to be fixed first, but the SeaFoam took care of the "leftover" issues from the carbon deposits on the valves/in the combustion chambers.
On the other hand, I ran a can through another car, that had had a recent head gasket replacement. The seafoam did NOT produce any smoke, because (I'm assuming) the head gasket failure did a pretty good job of steam cleaning the inside of the engine.
C.) There are MANY, MANY people on this board (including some of the most knowledgable and respected members), as well as in the "real world" who have used SeaFoam and been very satisified with the results. It is not snake oil. Are most of the $2.99 STP/Gumout/etc.... Fuel Injector cleaners a waste? Yes. There basically kereosene, with a few other things, and they don't really do much. The SeaFoam, when used through the intake, gives a clear indication that it is working.
Try it for yourself. Based on the tone of your comment, I'm guessing that you either 1.) never used Seafoam, 2.) Didn't use it properly, or 3.) used it on a car that either didn't need it or had other issues. Try it on an older high mileage that has no real engine problems, but maybe just feels a bit "off", maybe is down on fuel economy. Maybe it pings going up hills, and never used to. You might be surprised.
......just don't do it when the neighbors are home....