I use Seafoam all the time. It does work and works pretty well. I might be able to find the old pictures I took of a string trimmer I tested it on that were before and after pics from the very first time I tried it. I have been using it ever since. There are some other ones out there that work well too but I like Seafoam for my all purpose stuff. I can give lots of examples where it has worked for me.
Sorry, forgot to say that I use it on my diesel too. I used it most recently on my diesel that sat for several years. When I went back to it the fuel rack was stuck, injectors gummed up, and fuel lines gummed up. I flushed all the lines with Seafoam and ran it off pure Seafoam from the filter bulb. I also put it in the oil before I drained it on that machine. It runs great now.