I too have this problem.  I spent a couple weeks uploading all my CDs and all the music from 3 different computers into Google Play.  I believe that seemed to work ok, although I had A TON of doubles that I had to delete.  As well as a bunch of music that must have had incomplete TAGS, as the were not categorized.

Now that I took the time to get everything all straight, I want to sync the music onto all 3 computers by downloading the music back to each one so I have a complete collection each place.  I have all my songs in Google Play, but I cannot get them to download to any of the computers.  The best I seem to get is blank folders, or maybe a handful of tracks.  I let the Music Manager run for days with no luck.  It seems to reallllly slow my machine down too...this is something I wouldn;t want to run constantly.  But no matter how long I run Music Manager for, it seems to stay at "Preparing to download".