I'm tempted to get one now that I'm doing water changes 1-2 times a week on my 125g, but I've always been skeptical of adding water directly to the tank from the tap.
I've always done my water changes by adding the dechlorinator (using Prime now) to the 5g buckets and mixed it before adding to the tank.
Since you are adding water directly from the tap, how much Prime (or whatever you use) do you add to the tank? Just enough to cover the water being added, or do you need to add enough to treat the whole tank? Do you add Prime before/during/after the water is refilled?