They work like a charm.
You won't be sorry you got one. You can syphon out the water, then with a twist of the faucet attachment, you are filling you tank right back up.
When I do it, I just dose the tank with Prime (using the full tank volume to determine dose).
You can get an aquarium thermometer to hold under the stream of water coming from the faucet attachment in syphon mode, then just adjust your hot & cold water until you get within about 2 degrees of where your tank is. Once you gt to temp, twist the attachement and you are ready to fill.