I was suspicious of the stuff at first. But I got a can and tried it on my choptop a couple years back. Sucked part of a can into the intake via a vac line, killed the motor and dumped some in the gas tank. Let it sit a few minutes, started it and let it idle a bit. Shut it off and an hour or so later I went to leave. Talk about a smoke show... For the first quarter of a mile down the road there was nothing but a massive thick cloud of black smoke following me. Unbelievable the amount of smoke it was throwing out. Truck seemed to start better after that and run better without stumbling.
Naturally, I still wasn't convinced. I thought maybe it was something to do with whatever stuff SeaFoam is made of to make you think it got cleaned. So I picked up another can and tried repeating things the same way with my Ranger (at the time it had around 60k miles and was very well taken care of including full synthetic oil since aroudn 45k). Very little smoke and didn't seem to change anything.
Tried it with my F-150 (not sure how well the previous owner took care of the truck, I suspect it wasn't up to par), and got a good bit of smoke, not as much as the choptop, but the F-150 is supposed to have considerably less miles. Still have a bit of a hard start, but it seems to have helped the stumble.
I still don't think it's a miracle product, but it does seem to do some good and is not all hype. I recently picked up another can and have yet to decide on what test subject to try next, lol.
Oh, yea, I haven't tried dumping it in with oil, but I do use ATF in with my oil to clean things.