K.M.
we've had the Wii and Wii Fit for about a year and really like it but found we weren't using it as much because we couldn't do it together (one at a time vs. split screen). we got the Wii Active and that one allows two people to work out together and it's more of a workout vs. games that make you sweat (Wii fit does have yoga and other ones, but it's easy to just use it like games vs. real work out).
we haven't bought the part two of both games but think we might since it's only $20 for each and you can add to the exercises, routines, etc.
we found with the Wii fit that the kids got into it with us (two teen step-daughters and my two year old). I could pretty easily do some yoga and other exercises with my toddler crawling and walking around me while I did it.
Like everyone else said, it's keeping it going on a regular basis and managing the nutrition aspect as well; two things we've managed to not do well at all in recent months. but that's what new year's resolutions are for, right?
best of luck to you...we're getting back on this train as well this year.