A whole bunch of people at work are getting into it and we're having a lot of fun! It's $5/class here and I usually go twice a week. I don't think I'd be motivated if friends weren't going with me.
Also depends on the instructor. The first time I did it I hated it. The instructor was annoying and explained everything in ridiculous detail, going on and on about how she trained with the creator of Zumba and all these stupid stories about him.. drove me nuts! Now I have 2 instructors and I like them both but like one more than the other. Feels like a better workout with one, the other slacks a little.
AND a friend just lost 30 pounds and 4 dress sizes in a couple months doing an hour of zumba a day on her playstation at home. I haven't really lost weight but I only go twice a week.
/zumba ramblings.