SantanderCycles is very helpful for locating stations to collect or drop off bicycles. It tells you both the number of bikes available as well as spaces available to drop off.
However, it is of limited use for actually reserving and paying for bikes, at least if your party has more than one person. SantanderCycles will only allow one rental at a time, which means that all the time you put into setting up your payment method, etc. is really wasted if you have more than one in your group.
On a positive note, the kiosks at the rental sites accept cards and allow you to select multiple bicycles at a time. So download SantanderCycles for tracking and an easy way to locate bikes, but do your actual rental onsite!