I've been going twice a week, and I do think it has helped with my weightloss, but I think you could say that about any activity I would have done, if not for Zumba, twice a week for an hour each time, that got me completely out of breath!!
I'm putting the fact that my waist seems to be dissolving nicely solely down to Zumba and the shimmying and wiggling that goes on in that hour!!
Kam_khakh is right, it is very addictive! Although you may feel as though you've not had a full workout whilst you learn the dances, but next time you go, you'll soon remember them and then you'll feel the full Zumba effect!!
Enjoy!!