Ive experimented with DOF only a few times. The main reason is because the render options only allow DOF to be ticked if you have "premium" render quality ticked, which makes render times VERY slow. So unless you get it right 1st time, its a nightmare tweaking it.

What I did is create a very basic scene 320x240 with a flat grey sky & delete the ground plane.
Then add about 20 cubes with default mats and just randomly placing all of them into the distance. (only about as far as the end of the ground plane would be).
Then make 1 cube in the foreground (make it a different colour if you want) and whilst its selected, click the "set to current selection" button.

you could put a few cubes in the foreground too, but i havent really tried that yet.

This way, the render times arent too painful and you can see reasonably fast results whilst experimenting. hope this helps.