I just tried balance board on a whim tonight, and promptly plowed my way through the minor and major circuits with only one loss to Don Flamenco. I restarted once on Soda Popinski but just beat him by TKO in the 2nd.
Once again, reviewers have shown themselves to be uncoordinated spazzes. I know exactly what they are doing wrong on the board, and it's the same thing people did wrong on the headers and skiing in Wii Fit. The secret is to simply lean, which is actually harder than it sounds. When you go to lean on one foot, the natural tendency is to push off the other foot. The board reads this as you leaning and before you can get the weight onto the correct foot, you're already dodging the other way.
I got very good at leaning left quickly, but once I got to Great Tiger I realized how slow and jerkily I leaned to the right, and how my weight shifted left first. I practiced switching side to side, even when it wasn't necessary, just to get the feeling right. Soda Popinski really tested my ability to lean, center quickly, lean, center quickly, etc. And the hardest part can be getting centered in time to start punching.
In short, the balance board can work quite well. I don't know how well I'll do on Sandman or TD, but I'm more than satisfied so far.