What is cool about the Wii Fit is that it makes exercise fun (and in most cases more challenging which I mentioned before). You get to do yoga poses (I would never ever go to a Yoga class, but I don't mind trying poses in the comfort of my own home). You get to do strength training (without weights/resistance) so it is basic strength training (although it is by no means beginner, aka you will be down on the floor doing sit-ups and push-ups etc.). For aerobics there is hula hopping (very tiring), jogging (you can jog against your friends which is really amusing), dancing like Dance Dance revolution but dummed down a bit. For balance, you get to do ski jumps and ski hills, tight-rope walking -- all sorts of weird crazy fun stuff you'd never do at the gym. So, if you can imagine yourself, in your living room, standing on your Wii Fit board (which calls some people fat sometimes, by the way), doing some of these things, then yes it will work! If you want weight machines, and recumbent bikes to do exercise, then no, you probably won't like Wii Fit, so it probably won't keep you motivated very long.
By the way, Wii Fit is very smart about how it keeps you motivated. For every 10 minutes you exercise, it opens up a new exercise for you to try. Also, the # of reps and time on each exercise, and speeds increase as you get more and more advanced. This setup gives me a great sense of accomplishment. Also, more games will be coming out for the Wii Fit (I can post about them if there is interest), so there will be even more variety (I have no issue with variety today, but I could see possibly being bored after a few months).
So, I hope this was helpful. Please feel free to comment with questions or email me at firstname.lastname@example.org.
Enjoy your Wii Fit! It does work!