Every time you take a bike, they will charge you one euro for every thirty minutes you drive. You have a maximum of 24 hours.
You will find stops at many sites in the center of Rome. Apparently, Roman users are happy with the system: the bicycles are in good condition and the system is used to working properly.
It really is a good transport alternative in Rome. I remember that subway communications were scarce and buses or trams were stopped most of the time. With the bike you can make trips to less touristy places like walking along the river bank to the castle of San Angelo or even boarding the train and stopping in Ostia where a brand new bike stop was waiting for you to enjoy the beach.
For more information and maps on bicycle pick-up and return check the official website | ATAC Bike Sharing5.001