Gold's Gym is not bad, but as with all these games, you get what you put into it. You could easily "win" the game by sitting on the couch and just flicking your wrists
On the other hand, if you bob and weave (as they instruct you to) as you're throwing the punches, you actually do get a moderate aerobic workout. Still not the same as a real workout, but better than nothing.
There are some good exercises like crunches and squats which use the Balance Board (basically as a "spotter" to count the number you do). This gives you a decent workout.
I don't find the Wii Fit particularly useful for exercise, except maybe the Hula Hoop game and the boxing game. Still, I think it's only time before game developers come up with a lot of uses for the Balance Board...