When I got my iMac, I used Senuti. It worked fairly well in that all the music was transferred from my iPod to my computer, but it did have some issues with duplicating files. It seems to be linked to songs in multiple playlists, so if you have a song in 3 playlists, the song gets copied over three times. I can't say if that's the issue for sure, but that's what it looked like to me.
Sean