PlanetLab is a globally distributed overlay platform that has been increasingly used by researchers to deploy and assess planetary-scale network services. This paper analyzes some particular advantages of PlanetLab, and then investigates its evolution process, geographical node-distribution, and network topological features. The revealed results are helpful for researchers to 1) understand the history of PlanetLab and some of its important properties quantitatively; 2) realize the dynamic of PlanetLab environment and design professional experiments; 3) select stable nodes that possess a high probability to run continuously for a long time; and 4) objectively and in depth evaluate the experimental results.