First of all, what is Velib'? It's a huge bicycle sharing system that you can use all around Paris. It's similar to those you can find in other European cities (in France, Germany, Netherlands, Spain...) and, basically, all over the world.
So, how does it work? Well, depending on the fare you buy, you'll have blocks of either 30 or 45 minutes free of charge. After those minutes, you can either deposit the bike to a Velib' station and wait 2 minutes to retake it or you can continue but you'll be charged of an extra 1€ for the first additional half an hour, 2€ for the 2nd half hour and from the 3rd half hour, it would be 4€ each.
Now you might be wondering how much it costs, well it depends. There are the long-term subscriptions and the short-term tickets.
– Long-term subscriptions: they are valid for a year and it cost 29€ per year (30 free minutes each journey) or 39€ per year (45 minutes each). You have an unlimited number of journeys within that year. But, as always, if you are young you have some benefits. That would be: for the 30 minutes fare, you'd pay 19€ instead of 29€ and for the 45 minutes one, you'd pay 29€.
– Short-term tickets: they cost 1,7€ for a whole day or 8€ for 7 consecutive days. With this ticket, nevertheless, you only have 30 free minutes.
You might have two main problems: either there is no bike at the station you are or there is no free place at the station you want to deposit your bike. Well, a terminal can be found in each station where you can search for nearby stations in order to check whether there are free spots or available bicycles.
I'm going to give you a piece of advice: always check the brakes and the chain, they're the most breakable parts of the bikes.
You might wonder: is it dangerous with all the crazy Parisian traffic all around me? Okey, I admit it, it might seem a bit scary at the beginning but, if you are cautious you shouldn't have a single problem. There are a lot of Velib' users and drivers are pretty used to them. Moreover, in wider streets we can use the bus lane so you only have to share it with a few buses. In addition, in some places you can find cycle lanes that you can use.
One of my biggest fears before starting using it was being forced to go in a direction I didn't want because of one-way streets. However, most of one-way for cars, are two-way streets for bikes, so there are pretty few limitations for cyclers.
Now, what is my opinion on this system? I love it. I must admit you might not have high-quality bikes, that's true. Nevertheless, you get this dynamic fast eco-friendly tourist-friendly mean of transport! There are Velib' stations almost everywhere, specially all over the city centre, so it's not a problem fining one!
So, my final advice: use it! Discover Paris in a fun way while you exercise and forget about the depressing metro!