In your experience are there usually bikes available at docking stations? Yes.
Next question should be: Are you always able to undock a bike? No. Sometimes the key you paid £48 for gets you nothing but red lights, while people paying £1 at the ticket machine merrily take bikes from under your nose, and sometimes you walk along to the next docking station and the same thing happens again, so you give up and get the bus. Happened to me on Saturday at Bethnal Green Road and Commercial St.