My first time I walked into one, I was underwhelmed with the size of the place. They're typically really small with a few machines placed in a circle.
They do 30 seconds on each machine, a bell rings, and every switches to the next one. There is also some kind of step/board you use inbetween machines or whatever. There are some girls who just sit on their machines and chat, regardless of who they're annoying.
If you need to be surrounded by people to keep you motivated, then go. If you can do it by yourself, buy a fitness DVD.
As for the price... I remember that if you go into one and ask them about being a member but also being skeptical, they'll give you a week for free to try it out. I never went twice, so I don't know the exact monthly price.
EDIT: I've read some pretty bad reviews online for the company. They are apparently known to push products and have underqualified staff (I noticed that in my local one, too). The price is also crazy; it's about $50-$75 down payment and about $30-$40 for each month after that. Those 30 minutes may seem like a selling point, but that's all you seem to be getting everyday for those $30-$40 per month. My experience with them was negative, but it was mostly due to the staff and annoying chatters tying up the machines.
My opinion? I would suggest buying a few $10-$15 exercise DVDs instead, or going to your school's fitness center. High school with sports teams and colleges/universities are pretty much guaranteed to have one, and it's free to use for students.