The AC cord then plugs into this adapter. For a power cord, it seems like quite the deluxe item: it has a velcro strap for easy cord-winding, and a really bright blue light on the AC adapter part. Since the Balance Board is designed to sit directly in the middle of the room, and thus necessarily distant from walls which would house outlets, the Energy Pack requires a long cord. The cord comes in at ten feet long, meaning you can probably charge the Balance Board from its normal gameplay position in your room.
The only thing I can't speak about regarding this accessory is battery life. I've been using it for a day, and it has yet to die. Nyko promises 30 hours per charge, which sounds like plenty. Like all rechargeable batteries, I expect that this would lose charge capacity over time, but the ability to play while charging amelioirates those issues.
$20 seems like a lot to spend on an accessory for an accessory, but in the long run, rechargeable batteries are always worth it. It's a matter of personal preference whether you want to purchase a specialized unit for your Balance Board or just use rechargeable AAs, but it's certainly nice to be able to plug the Balance Board directly into the wall to charge, especially since you can do this while playing. If you're planning to use the board every day (as every Wii Fit owner is still planning to do, at least), it would be a sound purchase.