In Barcelona we have a very simmilar program. It's called BICING. Our bike sharing program was created before the Paris one.
As WestAust said:
1) everybody wants a bike to commute to work in the morning, so in the office district, the "parking" gets full quite rapidly with no more spaces available, which create a shortage in some areas during the day, and an overflow in others.
2) lots of vandalism to the bikes, graffitis, people leaving the bikes in the middle of nowhere and the police have to take them off the street and bring them back to a bike parking.
In our case the second point is not true. To take a bike you have to insert your personal "bicing" card, if not you can't take a bike, so it's impossible to take a bike without the card.
The cart cost 24 euros for one year. You can take a bike for half an hour, and if you spend mor time you'll pay 30 cnts. for each 30 minutes. (After 2 hours you will pay 50 euros...)
The Bicing service is open from 5 am to 2 pm, and Friday and Saturdays all day.
As you can see in the map there are a lot of different stops around the city, all of them perfectly combined with the metro stations:
Its a very good idea.