Hi. I've had an Xperia Z1 for over a year now, and the music transfer with Media Go has always been and remains nearly unusable. (And I do need it, because I have playlists for my music.)
- It is ludicrously slow. (The phone's been connected for over an hour, and "synchonizing from library" is still at 0%.)
- The transfer is not complete in one step. Sometimes, it transfers new songs, but doesn't update the playlists, and sometimes, it won't even transfer the songs in the first place, and I have to start over.
- The software doesn't always recognise files/playlists that are already on the phone. It then creates duplicates of songs (and of playlists - adding numbers with each duplicate).
- Backup using the Sony PC Companion is also very slow and often ends with an error message.
- EDIT: Disconnecting the device did not work, either. It was connected for several hours, it did not synchronise anything, and I couldn't disconnect or turn off Media Go without just pulling the plug on the phone.
I am very happy with the phone, but the desktop software makes me seriously consider switching brands! 
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