Some very complex ones incorporate drip irrigation to the planter. The down side to greywater systems is codes can be fairly strict in allowing there use. The main worry is that this used water can carry diseases and stuff which should just flow down the drain. Often the easiest way to appease code people is to build a traditional system and a greywater system hooked together at a diverting valve. That way it can be normal for inspection and switched to greywater whenever wanted.
Toilets are are slightly different.