For one, you can actually get a good cardio workout and burn some real calories with this game. From what I've seen, they have Excercise routines that can last up to 20 minutes!
Most importantly, this game is fun. The bulk of the game is a rhythm type game (like Guitar Hero or Donkey Konga) where you throw different punches at the right time. The motion-sensing recognition is good (although not perfect), and you get to use two Wii-motes instead of one plus a nunchuk if you so choose. This is good because you don't have to worry about whipping yourself with the nunchuk remote.
There are quite a few unlockables to keep you coming back...plus you can get a pretty good workout for those looking to get some exercise in the living room. Let me know if anyone else has played this game and what you think. I'll say that it's worth the 30 bucks, but I have seen it for as little as $20. Also, the Balance Board only adds about 5 or 6 exercises and it's not used for the "main" game, so don't shy away from this one if you don't have Wii Fit.
Thanks for bringing this game to my attention. Believe it or not, over five months after I bought it, I still use Wii Fit around 5 times a week (and have lost about 10 pounds). While I still enjoy playing it, it would be nice to have an alternative game to help spice things up a little. I may wait to see how EA Sports Active turns out, but the Gold's Gym game might be a good, cheaper option.
EA Sports Active will probably turn out to be good too, but I think it will be quite a bit different from the Gold's Gym game. There's probably more strength training in that game and Cardio Boxing is focused more on cardio. In my opinion, I see Gold's Gym: Cardio Boxing as a better way to shed a few pounds than Wii Fit. There aren't too many cardio games on Wii Fit, and after the novelty of hula-hooping and running in place wore out, Rhythm Boxing was the only game that I still played consistently (for exercise purposes).