I was a Jazzercise NUT for YEARS...and I'd still go were it not for my current crazy schedule.
I HIGHLY recommend Jazzercise - no matter what your weight or skill level (or in my case, klutziness level) pretty much ANYONE can do it, it's never boring, always changes, all the instructors I've taken classes from in the past have been extremely knowledgeable in fitness and health, AND I immediately felt accepted (even though I was still close to 200 lbs when I first started in 1990). In fact (and I don't want to get raked over the coals for saying this - not meant to be a flame) I would DEFINITELY recommend Jazzercise over Curves - or at least as an adjunct activity - because many of the reasons people sign up for Curves would also be reasons to sign up for Jazzercise - a women-oriented setting (there ARE some guys, but they're VASTLY outnumbered by women) of ALL sizes and abilities, a FUN workout, great music (which frequently changes - but still easy to keep up with!) plus you're not 'locked into' a contract or need to pay an 'enrollment fee'.
I personally think that one of the most critical elements of making exercise part of a permanent healthy lifestyle change is to MAKE IT FUN! And that's EXACTLY what Jazzercise is - it's FUN.
(and if you get really hooked - do what I did in 1991 and go to a Jazzerjam (Jazzercise convention) or regional event - they usually organize regional fundraisers for the Komen Foundation each year.