I did get to ride one in Prospect Park, but not the full loop. The other day I took one from 43rd St near my office and rode to 15th ST/PPW entrance, with a switch of bikes at Atlantic Ave as I was running out of time. I did the last stretch inside the park. This is the farthest into my end of Brooklyn that they go.
It felt strange to be on a Citibike in Park Slope as I've always just used them in Manhattan, except to cross the bridge and dock it at Dekalb Ave.