I've had mine for two weeks now and it's awesome.
I've got about 2/3 of my music collection uploaded so far. Buffering is fairly quick and it proactively buffers the next song in the list so you don't wait. Buffered music is kept offline for some unknown amount of time. It's super easy to "pin" music you want offline, so it's available offline all the time - you just long press and say "make available offline" and it downloads it to your SD card.
I now use it instead of syncing playlists or whatever. I just unpin music that I don't want anymore, and pin new music that I want. And I can keep my whole library available without using up my whole SD card.
Only bummer is that I had a few albums with the name "Greatest Hits" and it bundles all of them together. As in, if I'm in the "Artist" view and I select "Queen" then "Greatest Hits" - it also has songs from two other artists. No big deal, I just renamed the albums to include the artist names in them and I'm good.
Other than that... Sweet!