I've been running almost three years using a German unit , the AMH Booster 2, which I believe has now been updated to a slightly neater unit.
As some of the other units, it sits between the ECU and the common rail injector so has no detrimental effects on the management system. It certainly gives an increase in power and torque throughout the range and surprisingly give an additional 2-3 mpg.
As Geoff says, the idea is that it can be removed prior to service so there are no warranty issues.
At about ÂŁ80 at the time I bought it, it has been money very well spent. Also with the option of being transferred to any other common rail diesel with the use of the appropriate vehicle specific wiring harness available from the company.
information in English or German available from :-
shop@ihr-tuningpartner.de