Product review (5 stars): Helps add humidity to a small room
I use this in my son's room which is 10x10.  It does add humidity to the room overnight.  It's possible that it does restrict airflow from the vent, but it's minor.  Plastic is very flexible if stepped upon.  This is good because it sticks up higher than a traditional vent cover.  It must be refilled daily or more.  Much cheaper option that a traditional humidifier.  I would only recommend for small rooms.  It might not raise humidity in large rooms significantly.  Very happy with purchase.