Its main appeal, as explained by Piolet's developers, is that the application is optimized for all types of connections, from large bandwidths to slower 56-Ks connections.
Searching on Piolet is quick and easy, offering plenty of results and making full use of the connection.
The application itself is very comprehensive. It has all the most common features included on other P2P clients and specializes in MP3 music files.
In addition, it doesn't use a server (it's decentralized). With Piolet, you can play the files, create playlists, monitor CPU usage and memory, and make use of a long list of other features.