it doesnt change the quality of video but it adds many cool features that big camcorders have such as "zebras" to visually see where you are clipping right on the screen, there are audio level controls as well as visual display of them as well to make sure you aren't clipping your audio, there are several cool photo features ive used like an intervolometer and larger hdr bracket settings, and the list goes on and on.
so it wont make the image cleaner or clearer by itself, but using all of the added gizmos and gadgets that conveniently help with filming, you will find yourself making sharper images and videos because you are paying attention to the detail and it is helping every step of the way.
i have it on my 60d and absolutely love it