Well... You can't say that it does not alter the original picture, but if that is an improvement or degradation is up to the user. I used FFdshow with post processing before, and the result of low resolution videos was stunning. I did not only sharpen, but resize, sharpen, blur and so on. On things like DVD's it made a really good difference. Now on the other hand, I only use HD media, and are about to delete all my DVD images. So it's not needed any more. But if I still was into DVD's, avi's and xvid or what not, I would not hesitate to use post processing.
Try to look up some FFDshow post processing guides to get a push in the right direction.