I have wii fit plus. It is alot of fun. For someone who works out alot i would say no, the workouts aren't really that challenging, but it does work alot on form and balance which is always good. Plus, it has a variety of different things to do and try. For kids and people who are newer to fitness i would say yes, some of the exercises would be challenging.
One good thing: As you get better at it, new challenges are opened up- like new exercises and higher rep patterns on old ones.