When I purchased the Wii Fit, I also purchased this pack at the same time.
It's definitely a good value, since the Wii Fit probably drains AA batteries quickly. I've used mine for 9 days with no problems. As long as the battery pack is charged, I can unplug the board and use it as far away as I'd like.
Unfortunately, the USB cord is pretty short and, in addition, the recharge pack covers the Sync button you use to setup the board.
Overall a good value, but those issues are minor annoyances.