I have a few friends who do this sort of racing, they seem to be really into it. I gave it a try once, it seemed much more fun and laid back than what I was used to.
It may be a good introduction to racing for younger folk, but the skills necessary to successfully get one of these tiny bikes around a tight road course might not necessarily transfer over to larger bikes on a full sized road course. I've seen a few people struggle while making the transition.
At the same time, I think both Rossi and Haga started on minimotos.
Either way, it's a fun way to spend an afternoon out with fellow competitors, at a mere fraction of the cost and risk involved with full size motorcycle racing.
Check this site out for more information, if what you are looking for isn't on there, it will likely have links to whatever it is you are looking for:
http://www.teamcalamari.com/
Have fun,
Mike