I know there was a discussion about this recently, but I wanted to do some more tests, and even go "all out" on one of them.
I rendered one at quality of 0.6, another at 0.8, and then another at 6.0. The 6.0 does look better and seems to have a bit more depth, but the render times were incredible (see file names for stats). The best way to compare them is to have the imags in a folder and switch between them in an image viewer. You can definitely tell the difference. My opinion is that the higher numbers ARE better, but the benefits drop off quickly as render times exponentiate.
More tests to follow.