I usually use long lines when fishing salmon. The little bit of electric voltage that comes off of these black boxes or down rigger cables doesn't travel that far, unless your tight to the ball or still fishing. Save your money for electric downriggers!
If you want to give the black box a try, I have one I'll sell you cheap. Its been on my boat since I bought it, hooked up to four downriggers, but I've only used it once or twice. Honestly couldn't tell you if it makes a difference or not. I catch quite a few without it, so never felt the need to bother with it. Send me an email if you want it. Only bad thing is that its on the boat and I won't have access to it for a while.
