Its just for collecting rainwater off the roof all a bit environmentally driven, however I have hundreds of rain forest ferns orchids and plants  I propagate and enjoy
it generates more maintenance tasks than anything else.
So if anybody tells you it will save money I think they are a little misguided as,

The negatives:-
1. They take up grounds space.
2. Require cleaning out on a regular basis.
3. Tend to be a bit of an eye sore.
4. Require the existing storm water drains to be modified.
5. Roof gutters need constant cleaning.
6. A pump and additional hoses to distribute the water
7. It uses more electricity to operate
8. No moonshine is produced in the process

The positives:-
1. You get to visit your garden on a regular basis
2. You do not use as much council metered and costed water
3. Its good exercise and enjoyable pastime
4. You piss the neighbours off when there are water restrictions in place (snicker)
5. Underneath provides a home for my beloved Water Dragons and Blue Tongue Lizards that love eating cockroaches and annoying insects.
6. You think your doing your bit for the environment.
7. You get to makes lots of wooden "things" that get used in the process
 
In the final analysis the only thing you "save" is the water from just going down the drain.
Anyway back to woodworking!

Regards Rob