I've used Seafoam a number of times on different vehicles and always had good results.
The way I see it, the cleaner the engine the better!
I used seafoam on my old VX at 110k and there was a HUGE smoke screen of white smoke. lol. More smoke than I would have expected. But after I ran it all out and changed the oil it ran smoother than it did before. The idle was improved, the acceleration was improved, and I'm pretty sure FE went up a bit, but I don't have my records any more since I sold the car.
In my opinion Seafoam is definately worth it, especially on older cars.