Don't know about the snake oil comment. The comment about needing to rework the injectors is probably valid.
I can tell you I did a DP on my 84 300SD yesterday and there was a noticable improvement in the engine running, it idles smother and isn't as noisy as before. How long it will last I haven't a clue. I do intend to either rework the injectors or replace them but the DP did make difference.
Darrell