It seems that while the Wii's boxing game isn't vigorous enough to provide a comprehensive cardiovascular workout, it can play a role in maintaining or losing body weight, especially for someone starting from a lower level of fitness.

"If they're looking for cardiovascular fitness, I'd advise them to do something else because it's really not intense enough," White said. "But if they're just looking to lose weight, it's a good way to get started. It can also be a gateway to other things, and may get people interested in the actual sports themselves too."
Boxercise

The most energetic workout of the three trials was the boxercise video, while walking provided less exercise than the Wii.

But White's report, which he's considering submitting for publishing, noted something interesting: study participants who were gamers to begin with had a lower level of perceived exertion playing Wii boxing. In other words, it felt less like exercise because they were enjoying it more.

I guess I need a HR monitor to know how many calories I am burning during a 2 hour Wii boxing marathon.