The dealer used it on my 1986 420SEL last year when I couldn't pass the emmissions test. I had alot of carbon build up and was getting some pre-ignition. Motor vac cleared it up, and I passed emmissions. This really just cures symptoms, it does not fix problems. I don't know if the chemicals are harmful to seals over time or not, but I would warn against using it often. Probably once in the lifetime of the engine is enough.
1995 E420 - retired
1986 420SEL - retired