Very fun. Not terribly effective.
For me, it had an average lifespan, about the same as any other Nintendo title on the Wii. It has a good mix of activities and exercises, but I don't think any of them really get you going at a level to be truly effective. Some possible exceptions are the hula-hoop and the step aerobics.
One of the biggest advantages imo is that once one of you starts using it, others want to play too. I may not be motivated to play it, but once I watch someone else for a few minutes, my competitive nature kicks in and I want to give it a go. It can be really fun in that regard as long as everyone can remain friendly about it.