During normal driving conditions the ECU will compensate for the lower MAF output voltage of the CDI MAF. If found that the ECU closed the EGR by an extra 10% at 120km/hr (75mph). This way the ECU regulates the actual MAF value to the expected value. So the engine gets more fresh air and less recirculated exhaust gasses. As this will result in more NOx, this may be an environmental issue. So beware!
So how good is it? It is pretty close, and I cannot tell the difference when I'm driving. (81kW TDI) The readings are not perfect of course, but it is much better to use a MB CDI MAF in a TDI than a VW 2.0L MAF. The 2.0L MAF produces really high reading which may result in smoke.
Update Sept 7, 2003
Before doing more measurements, I wanted to see the MAF output signals on an oscilloscoop. This showed some remarkable results.