When I first encountered Diesel Purge, I used it on my '83 300CD-T and received good results. Specifically, the engine ran quieter (less knocking) and I noticed a small improvement in acceleration. This was done at a point where the engine had about 200,000 mi. on it.
As for application procedure, I have a diagram that came with the product that I have referenced and which involves a piece of clear tubing and a jar. About 20 minutes is what it takes me with a helper in the car controlling the ignition. I generally use the product on each M-B once per year.
Of course, your results may vary.
Incdientally, I also add a small amount of Red Line's Diesel Fuel Catalyst in to the tank at each fill-up which seems to successfully eliminate smoking (I think "the patch" placed by the fuel tank door would detract from the car's aesthetics... :> )