I got a Wii Fit in July. I was noticing my weight go down slowly, and then unespectedly found myself pregnant! I really love it. There are some things that are open (aerobics, yoga, balance) and as you work out, you "earn" others - advanced balancing things and more "games". My favorite is the boxing. I LOVE it!
You use your Mii and every day when you get on the balance board, you can check your weight and BMI and it will chart your progress for you. You set all that up when you first start. Also, my 8 year old loves to play it, too. And, when I'm happy jack about something, I can usually get my hubby to "play" the games, too.
My plan is that when this baby is born in April to start right up with my Wii Fit again as soon as possible. You make progress with the weight, but you compete against yourself and it's more like playing games than exercising. And, I get tired of exercising - it feels like an obligation - my Wii Fit is fun, not an obligation. So, my vote is go for it! They aren't always easy to find, but I just love mine. I seriously was anxiously waiting nap time each day so I could get back to Wii Fit. (Kinda hard to do with a then 2 year old running around!)
If you have other questions, let me know - like I said, I love mine and am excited about getting back to it in a few months!