If you aren't an idiot, and you don't just stick the vacuum line in the can and wait, you'll be alright. Put 1/3 can in a cup, tilt the cup sideways and slowly feed it into the motor via a vacuum hose.

The way I see it, if your engine needs it, then it'll probably help. My friend just bought a car with 226k miles and the Seafoam treatment increased vacuum by about 2 inHG.