Experience? I've used it. I used the engine flush once, to clean things out when I have a "dirty" oil analysis. It was somewhere around 100k miles. I didn't notice anything substantial happening, and didn't expect to, but it's cheap insurance to try cleaning things out now and then. I don't intend to get rid of my truck anytime soon. The PI is used in the gas tank, more with the initial treatment, and then a "maintenace" treatment everytime you fill up until the bottle is gone. Should do the same thing as the Seafoam, only takes a little longer, and get the injectors as well. I don't have any complaints.
Amsoil products are expensive, but I feel that they stand behind them, and I have never had a problem. I use Amsoil throughout my vehicle, and all of my vehicles now. '92 Lexus SC400, Amsoil XL7500 and Amsoil ATF. '05 Sienna van, Amsoil Series 2000 0W30, and soon to be Amsoil ATF. '97 Explorer, 100% Amsoil in everything - engine, tranny, transfer case, front and rear diffs, power stearing, and radiator.
I'm no engine guru, but by "upper" cylinder I'm sure they are refering to where detonation takes place. If you get oil in there, then you're buring oil, get a lot of deposits, and have other things to worry about. The oil doesn't get to that area, which is why you use PI or one of the others, which gets there the same way the gas does, by being injected. That's why it cleans the injectors and the upper cylinder and valves.