My drives run ~10C cooler with the ducts in place, although my MB now runs ~3C hotter since the air passing over it is now passing over the drives first. I'd consider this a perfectly fine trade off.
I have the "Nidec Servo Gentle Typhoon D1225C12B5AP 120mm 1850 rpm Silent Case Fan" and I found I had to manually trim about 1cm off the top of the duct for the drive bay to still fit in the case with the duct in place. You could probably make the ducts that much shorter by default to accommodate more fan types and still get good results with thinner fans and the small gap.
One of the screw holes on each duct broke while I was attaching them to the fans. This was probably because I had to remove the ducts to trim them and that added extra stress to the bracket. I'd suggest future printings to add more thickness to those areas.