Zumba is the best work out I have every done, just in the fact that it has me coming back every week.
I do atleast 3 days a week and everyother week I do 4, if there were more classes offered each week I would go 7 days a week.
I feel the work out in just about every area, arms, legs, butt, abs and back. The intensity is really up to you at first its difficult to learn the steps but once you have the basics down you can really start to add in more complex movements. (ei. a plain shuffle compared to a shuffle with hips added in)
For the the intensity is just right, I sweat and push myself and can still make it the entire hour.
And the best thing about Zumba... Its just fun.