I did Jazzercize this summer when a rash of instructor injuries canceled many of my aerobics classes at the Y. I'd say Jazzercize is basically an aerobics class. There are high impact moves, which you can modify, and there were a couple of low impact classes. The classes incorporate free weight training and stretches. I brought my own weights and yoga mat but weights can be provided.
The instructors were well trained and the music was motivating. The workout was intense. I've done aerobics for over 30 years but i'm in my late 60s and have the knees to prove it. The one thing that was a problem for me was that the classes were at a strip mall and the floor was linoleum on cement. That was murder for my knees and I actually had to go to water aerobics at the Y for a couple of months until my knees returned to their normal pain level.
I'd say if you're younger than I am and enjoy a good workout with friendly people, good dance music, and are already at a decent fitness level, go for it. If you're just starting out, take it easy and build up to a higher fitness level, which you can do at Jazzercize by discussing with the instructor how to best begin. Bottom line..have fun but stay safe!