I thought it'd be great exercise but you only need to lightly swing for boxing and tennis. My other half played on it lots but that's all old hat now. It belongs to the boys. 
It really depends on how you use it I think
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, I'm doing the My fitness coach: Cardio boxing and it seems to be quite good exercise, I really throw myself into the punches and with the feet movement I get a pretty good work out.
The key with using the wii to keep fit is to have variety, wii fit itself was quite dull and I seemed to master stuff easily, but then I progressed to My Fitness Coach, and there's also EA Sports Active which does more thorough work outs with resistance bands.
It's pretty much like anything else though, have you ever bought a yoga DVD and only did the routine twice? If so there's probably little reason to buy a wii & wii fit.
I quite like the weight tracking, I have about 3 months worth of my weight fluctuations up there to see, and it's really useful for me to see that line trending down, it's a great incentive.
There are lots of other good games for the wii that aren't just kids games like -
Okami if he likes Japanese stuff
House of Dead Overkil - if he likes B movie/pulp fiction style uber zombie violence.
MadWorld - if he likes to chainsaw people in half in black and white
World of Goo if he's into puzzles etc.
Bloom Blox is a fun family game
Legend of Zelda : twilight princess is just awesome
Super Paper Mario
deBlob
~There's just so much rubbish for the wii and people don't really know what they're buying, or they buy shoddy ports of great games on the PS3/360.