Does exactly as advertized. The documentation is very clear and complete. You'll never use RRDs again ;-)
The interface stays very close to rrdtool behaviour though, don't expect it to DWIM if you don't know rrdtool very well.
The criticism I have is concerning the structure. I'd like to see this separated into RRDTool::OO::RRD and RRDTool::OO::Graph. For instance, if you only want to graph, you need to create an RRDTool::OO object associated with a dummy rrd file, or one of the files used inside the draw elements.