Well, I hadn't quite thought of it the way the links portray it, but those clearly would work.
We live right at the top of the "flood zone" limit, and the soil is about 8" of dirt before going to concrete like clay. I've gone 36" down, and still in the clay, so I don't know how deep it is.

My idea was to build a heavy duty wood box next to the downspout, maybe 2'-3' tall, and 3' or so long, by about 2' wide. Then line it with something. It would have a lid that could be closed and locked, and a tap on the side. Of course, the lid could be opened and water scooped out. It would also be easy to clean if necessary.
I have these gutters, so not much debris: http://www.leafguard.com/

Those tanks are way too big. The city would almost certainly refuse the building permit I would have to have, for no contractor would dig the hole without it. I'm certainly not going to dig such a hole with a shovel. Also, the idea is "gray man". A contractor digging up half my backyard and sticking a huge tank in the hole will not go unnoticed. A couple wooden crate sized boxes, at the back corner of the house, could be explained as yard storage. Inspectors used to just drive around, looking for infractions, too lazy to get out of the car. A couple years ago, that changed. They will come up in your yard and look for things.
Our city has not (yet), but one local city has banned the collection of rainwater, or so I've been told.

I'm talking 100-300 gallons of water.