Unlike two thirds of the armchair respondents to your question;.... I actually have used diesel purge!
On a 240d it smoothed out the idle significantly;... I had to turn down my idle screw a lot because it started idling at 1500rpms from 700rpms and now it starts much much quicker and accelerates smoother.
No noticible power gain.

I was impressed & i don't impress easily.

(Diesel purge will not adjust your valves, or replace your motor mounts, or reset the injection timing, or shrink a stretched chain.)