It will smooth out rough idles a bit but I'd suggest putting new spark plugs in after. It fouled mine out, but it ran realll smooth afterwords.
I have never heard a first-hand case of damage from using it in the intake. People tell stories of it, but that is probably just propaganda from the skeptics. Using it in the oil is riskier but is not going to kill a motor unless its sludge prone, in which case you might develop a problem.
Fun story: When we did it in a friends car, I was pouring it in and he was keeping it alive. I told him to turn the car off. He held up the keys in his hand. The car was off, but was dieseling off of the remaining seafoam in the intake.