Peer Exchange (PEX), in which peers directly exchange with each other lists of active peers in the torrent, has been widely implemented in modern BitTorrent clients for decentralized peer discovery. However, there is little knowledge about the behavior of PEX in operational systems. In this paper, we perform both passive measurements and Planetlab experiments to study the impact and properties of BitTorrent PEX.We first study the impact of PEX on the download efficiency of BitTorrent. We observe that PEX can significantly reduce the download time for some torrents. We then analyze the freshness, redundancy and spread speed of PEX messages. Finally, we also conduct large-scale Planetlab experiments to understand the impact of PEX on the overlay properties of BitTorrent.