Some people say it works wonders. Some people say it's a waste of money. Personally, I've had mixed results, but mainly nothing.
Putting it through the air intake system never really did anything but create a bunch of smoke. I never noticed any performance improvements. I was more afraid of fouling spark plugs than anything else.
Putting it in the crank case, driving it for a while, and then immediately changing the oil only seems beneficial if you're using crap oil. I suppose this would be a good idea if you're putting it into a vehicle that has sludge buildup, or has been neglected. If you change your oil regularly and use synthetic, it seems like a waste here too.
The only other place I would consider putting seafoam is in the fuel. I don't see any harm in it as a fuel additive, and it would help stabilize if you got a bad batch of gas in the tank, but then again, so would sta-bil or dry-gas.