[I have two of them. They work great as long as you get one that will dose the maximum amount you need. For Example, I have one that doses a max of 295ml/hr. Thats roughly about 2 gallons a day. This is fine for my 55 gallon but not my 90 gallon. I have another that doses 999ml/hr. This is a round 5 gallons a day which is fine for my 90 gallon.
I recently bought a reeffiller off of ebay because it was a good deal. I no longer use the 999ml/hr doser. Sometimes I use it to acclimate but thats about it.]