Canon 500d

[puyjapin]

Has anyone used this on an existing lens? Does it physically increase the apparent size of aperture to make a shallower DOF?

 

[Overread]

far as I know it does not alter your camera aperture, what it does it to reduce the minimum focusing distance for the lens on the camera. This allows you to focus the lens much closer to a subject, on the right lenses the amount of magnification you are able to get is akin to proper macro magnifcation - though I am not certain what lenses it works best on.
At a closer focusing distance of the sorts used in macro work your depth of field at a given aperture will be much smaller - thus you need smaller apertures to get the depth back - think around f13. That does mean that one needs either a tripod for a long exposure (and a static subject) or a flash to add more light to a shot to allow for a good exposure and faster shutter speed (needed for handheld shooting)

 

[Samanax]

Has anyone used this on an existing lens?

Here's a quick review. Note: The 500D is meant to be used on lenses with a focal length between 70mm and 300mm.

Does it physically increase the apparent size of aperture to make a shallower DOF?

Aperture isn't the only factor that affects DOF, it's just one. You have to consider the focal length of the lens used, the distance to the subject, the distance from the subject to the background as well as aperture size. Depth Of Field Photography Tutorial.