I also go to Curves! I like it a lot, there are women of all ages and sizes there. I paid $74 for registration, but got the 1st 2 months free. Starting September 1st it will be $29 per month, a 12 month minimum.
The machines are not electric in any way (at least at mine) they are all powered by your body. We do not have a treadmill, I am unsure exactly what a circuit machine is. Some you squat on, some you row, some you use you legs for. At our Curves, you do 2.5 rotations. Often, I do a few extra machines!
Also, besides the aerobic pads and steps (you may do whatever you like on these for a "recoup" for the next machine), there are also jump robes and hula hoops (LOVE them

). You also check your heart rate every so often to make sure you are not woking too hard or too soft for your age.
Our Curves always has games going on, we are just finishing up a bingo game and are also playing a Color of the Week game (wear the color, get your name entered into a raffle). That gives you incentive to come back everyday to get more bingo numbers/entries! You can win Curves merchandise.
They have lots of motivation around you, a big board with names and weight/inches lost, a 20 lb or 20 inch lost board, a Curves Elite board (losing a whole lot!) etc. There is also a stretching area, for after the workout. It gives you several ways to stretch to cool down and improve elasticity in your body. I always do this and have seen an improvement over the last month in how flexible I am! They have those exercise balls there too, though I have not tried them yet.
Basically, the workout is about 35 minutes, plus stretching time etc. They sell books on the Curves diet, protein shakes, and we also have a book exchange, like a mini library. It is a very supportive atmosphere, it is not just some girl sitting at a desk talking to her boyfriend while the clients workout! The women who work there are very supportive, know your diet plans, and correct you if you are not getting the most out of the machines.
It reminds me a bit of the WISH family here, but Curves could never be quite THAT good
Well, as usual, I typed till my fingers were sore, hope this helped!