I have to agree, I think that this cleaning is a really bad idea.. if you have ever seen the inside of a sooted up inlet manifold you wont want the lumps that will surely break off going through the engine.
I'm also quit skeptical that it will actually clean it at all, it can only be just a mist that is injected and all this talk of mega psi, that is just nozzel psi just like an aerosol can and as it is done on tick over it wont touch the second port and swirl valves as they are closed below about 1800rpm.
and as for cleaning the inside, wouldnt an engine flush and a cheap gallon of oil used before the proper oil do the same job.
"To get into the manifold means it has already been through the engine" well sort of but it has come out of the exhaust over probably 50k miles or mor and now your sending it back through all at once
I think if the manifold does need cleaning you'd be better off putting the money towards a new manifold, after all the swirl valves will most probably be worn on most early cars anyway.