I have one and I absolutely LOVE it - I use it on days when I am short of time to go to the gym, if I only have a quick half hour spare.
However as above it depends on what you do with it, a lot of the activites are very slow paced, and stop/start and I dont feel that they help much.
Not the case (for me) for the Super Hula Hoop (10 min length one - once you unlock it) and the Rhythm Boxing (12/13 min one, again once unlocked) - these do get my heart rate going and get me sweating, puffing and panting - always a good thing. I also usually start with 2 rounds of the Step Plus (I think it is) to warm me up.... it gives you 30 (almost) solid minutes of cardio...
Then if I have extra time I might do some of the other games/excercises - Kung Fu and the like, and for some strength training the Muscle Workouts - however I find these MUCH more efficient at the gym with proper weights.
I have recently received some Dance Workout game also (which uses the Wii Fit board in places) and I am currently LOVING that one also.... that makes up a range of excercises for you for each time you go on - and it goes straight from one activity to the other (little time to stop and slow your heart rate) and it also includes a warm up and cool down automatically. Fun!!
That said, I use this as a back up to the gym/swimming/classes as I find the more variety I have the more likely I am to keep motivated and keep going back