Senuti allows you to transfer content from your iPod to your Mac. It was designed to let you get your music back in the event that your Mac suffered a catastrophic failure that erased your music library. Since your Mac syncs with your iPhone, the iPhone will always have a copy of your entire library (if you sync it all regularly). So Senuti comes to your aid when you need to download those songs again and create a new library.
When you launch Senuti and your iPhone is connected, it will detect all the content on your phone and display it in playlists similar to those you see in iTunes. However, you only seem to be able to retrieve music. The music is shown in a list with all the information that you have about it: name of the song, duration, album, artist, genre, rating, etc. This makes it easy to decide whether you want all your music back or just some specific songs.
Although Senuti was designed to let you back up or recover your songs, it also works well enough if you want to use it as a media player and playback all the songs that you have on your iPhone. Songs play one after the other and quality is really good.