I used to attend Zumba three to four times a week with an ol' friend of mine over a period of about four months. I was also one of only four guys in the class.

Anyway, it's a great workout and it's FUN too! At the time I was able to pick up a few nice Latin dance moves and lost about 23 pounds in a little over three months without dieting. Take in mind... I was building quite a bit of muscle too.
Some moves were hard on the knees. All in all, it depended upon the instructor. Some instructors focused on more bouncy and lunging moves, while others focused more on other moves. One instructor in particular would always leave me pretty sore. But even then it wasn't all that bad.
I say go for it.