I've been trying to find a way to get playlists synced from Winamp to Google Music. It was exceedingly slow, but winamp export playlist, itunes import playlist, restart music manager and change source to itunes does seem to eventually work. All the tracks and the playlists have appeared in Google Music. However, my largest playlist says 1000 tracks while it's 5150 in both winamp and itunes.
I tried this via Windows Media Player but although the playlists appear in WMP, they're not visible in Music Manager. .m3u is a well understood playlist format, so MM really ought to just find them in folders but apparently not.
Most annoying is that it has to be redone each time I update the playlist which is not a lot different from syncing an iPod in Winamp, but the whole process takes several hours, if not days so it's pretty horrible.
I'm rapidly reaching the conclusion that Music is yet another Google work in progress. It may eventually be ok and usable, but right now there's too many gotchas and unfinished or missing function.
And yet, I've yet to find anything as convenient and useable as winamp in the home and the iPod on the road. Google Play is a long way from this. It's also a long way from last.fm or even the Amazon store.