I have a 10-24 aspherical DX lens that I use on my D700 and the vignette kind of adds a nice touch to the pictures, it makes them completely round which is very different.
@ Spraynpray, I stand to be corrected on all I post here.
Q) "Perhaps the deeper-than-FX depth of field of DX means that the format really isn't suitable for pictures where the subject is heavily isolated using a larger aperture?"
A) If you use the right combination of lens, aperture and zoom, you can get good variance in depth of field with both FX and DX. The difference between FX and DX is the size of the sensor. The FX is "true" in that a 50mm lens is just that, a 50 mm lens. In a DX camera, a 50mm lens is approximately, depending on the camera, 75mm.
It is not a depth of field thing, it is more a magnification issue.
Q) "Can someone tell me what happens when you use the DX facility of one of the FX cameras? Is it just a crop of the image size, or do the (for me) above benefits of DX become available to an FX user?"
A) To the best of my knowledge, it is just a crop, and you end up losing a large part of the file.