Are air curtains effective?

Absolutely!

All studies and tests have proven that air curtains are effective. When an efficient air curtain is well installed it save lots of energy and at the same time the entrance areas remain climate protected, comfortable and free of draughts, insects, odours, etc.
The following advanced Computational Fluid Dynamics CFD diagrams made by UPC (Polytechnic University of Catalunya) show clearly the efficiency of air curtains.
The surface of next diagram is divided in two parts (at X=2): left side considered inside has a heater on the wall and refrigerated right side simulates outside.

Situation without air curtain:

Following the laws of physics, the light heated air on the left of the image, goes up by natural convection and escapes to the cooler side while the heavier unheated cold air penetrate from the outside flowing through the floor. As the air tends to equalize temperatures and pressures and the warm inside are not thermally protected from the outside, the air mixes and we can’t see two separate thermal areas. In this situation we waste lots of energy and the heater is not able to keep comfortable the internal area.

Situation with air curtain:

The air jet of the air curtain placed on top, clearly divide the warm and the cold area in two thermal zones. The air curtain keeps internal environment on the left side of the image at comfortable warm temperature and avoids cold air penetration from the outside. In this second situation, the air curtain saves up to 80% of the energy given by the heater placed on the left wall.
Important Note: Poorly designed air curtains, wrong selected units (less powerful) or inadequate installation will loose part or even all the advantages and became worse than unprotected door.