Here are my impressions of the Zalman fans.

No scientific data, just my own experience.

On default (12v) speed they move a good amount of air but are what I would consider loud in a case. However they come with a resistor that you can place in-line to slow them down from 2000rpm to around 1100-1200rpm which mostly silences them. The airflow is quite diminished but they are very good for building a quiet system.

I am not sure that they are much better than the NZXT 120mm case fans that I removed though...those are surprisingly good and upon inspection look almost like a Zalman clone.