Product review (3 stars): Something the other reviews don't mention....
Does add some much-needed humidity.
However, this product seriously blocks the airflow coming out of the vent. I have this in a small room which normally gets uncomfortably hot when the furnace runs (the vent is close to the furnace). However, with this product installed, it does not heat up enough. I have dry skin and the winter makes it bad so it is still worth using it to me.
My estimate from feeling the airflow with my hand before and after is that this product reduces airflow by at least 70%. The actual area for air to flow through is less than half of a regular register due to the water reservoir. The wick further blocks the airflow.

I would not recommend this for rooms larger than 10x10 unless the room has multiple registers. Get a full-house humidifier instead.