Well, I've got one but I always forget to turn it on. I would suggest doing a search on "Piscatorial Pursuits" discussion board under "black box" and you will find a great deal of information and opinions regarding the use of the Black Box. You may have to search the 2003 archives because I can't remember a recent discussion taking place regarding this topic.
The existing current (without the box) to my downrigger wires is pretty much in the recommended zone as it is, so I only make minor adjustments when I remember to turn it on and haven't noticed any appreciable difference using or not using the box. But, it's a glass boat and the zincs and wiring are in good shape. From what I have heard, fisherpeople using aluminum boats can really benefit from the black box. Hope this helps. Tim