Although I have not used one, I was told that the rotating vane anemometer isn't that good. Apparently the bearings will not be as free after a spell and the vane will not spin at the right speed; Thus the reading is off. At first I am sure it will work fine but I would question the long-term accuracy.
Perhaps one to look in to is the solid state anemometer by Testo. It is compact, runs off watch batteries, and appears to be something worth checking in to if you are looking for one. The probe is only about 3/8" diameter, thus will easily stick into the ductwork to get the airflow measurements. I believe the retail price is about the same as the "buy-it-now" price on that rotating vane model.