I lived there briefly and have visited a handful of times. I have been known to argue that Barcelona is the greatest city in the world. And Bicing is, accordingly, pretty cool.
Caveat: It's only available to permanent residents. Being there on a student visa, I couldn't access the bicis. The reason for this is that it's designed to actively resist being painted as a touristic or recreational novelty (i.e. toys interfering with traffic or whatever people might whine about). Its singular goal is to serve as an integrated part of the public transit system, specifically a part that is most convenient and useful for residents making small, routine trips. (Què no és Bicing?: El Bicing no és un sistema públic de lloguer de bicicletes per a ús turístic o recreatiu. El Bicing és un mitjà de transport i un complement ideal al transport tradicional de la ciutat de Barcelona. La seva finalitat és cobrir els petits trajectes diaris que es fan per dins de la ciutat... blahblahblah)
But... no worries, there are multiple tourist-oriented bike rental places in the city, if you want to ride when you visit. I lived there before I got into cycling, unfortunately (and keep meaning to go back and re-learn the city on a bike), so I can't offer personal experiences with any particular service, but I understand there are a few good options (and I can try to recall which those are if I think about it a little longer). Here's the city government's list of bike rental services, but off the top of my head I can't remember how one might find the English version of this stuff.
Also, if you have any general questions about what to do when visiting there, let me know.
Gratuitous photo of bicis!