why?
"I did it because I wanted to see what it would look like. You should be greatful that I even told you that it would look good. If your to dumb to use photoshop and change the color of your own car then you don't need to see it changed. I'm not going to give my work away. I'll sell the pic to you though for $15."-Adam Kalin
^^JBO is about not being that guy
I don't really understand what we're responding to? Do you want our opinion on if it's useful, looks cool, too expensive on ebay, worth buying? If you really want people opinions make the effort to make a real thread and you might get responses better then "why?". Honestly, I would have to ask that as well. Why would you want that? Install a volt gauge in your car or in your trunk next to some subs if you're worried about it.
http://www.cardomain.com/id/Gerkn
Not really useful. Think about where your battery is located. Unless you have it mounted in your cabin (bad idea) then that readout is pointless. Spend another 20 bucks and get a volt gauge for the cabin.
I notice it mentions "auto activation". I wonder how foolproof that is, and whether you'd end up with a crappy low-efficiency LED running down your battery constantly.

Shop Manuals, Brochures:
www.kenmcgeeautobooks.com
looks like a huge waste of money to me. especially for 40 dollars just go to glowshift but a volt gauge for like 30 dollars shipped and then buy a gauge pod from autozone for like 15 bucks.
there you go you have something that wont run down your battery, probably a better reading, you can actually see it, it will be like 50 cents cheaper, and you have a one year warranty on the volt gauge too.
plus you have 7 color changing and like 3 or 4 different color faces to choose from. imo thats a better offer.