My Humble Opinion
I think ducts are great. With the wide difference in airflow patterns due to unique case design and varying PCI card sizes etc, it is nearly impossible to predict what effect individual fans will have on the overall cooling effectiveness.
A properly designed and built duct virtually assures you that the airflow will be moving from Point A to Point B.
If it works out well for you I might try it myself the next time I rebuild my system. For my current system, temps are OK, and since I am only mildly OC'ing my T-Bird 1200 (to 1400) I probably won't mess with it until I can afford to upgrade to a Barton.
Prof