This thing is full of bugs.
I had issues with Google Play Music, since many songs experienced random stutter on my Samsung Galaxy S4 only, so I was looking for alternative players - and for a method to painlessly transfer music from iTunes to Android.
doubleTwist seemed to fit the bill, but only thorough testing proved me wrong, sadly after buying it.
You sync your device with a custom desktop client that accesses your iTunes library (or any other folder) and shoves your music to your device either by USB or WiFi or - if speed is needed - directly to an SD-Card.
WiFi worked out of the box for me, so that's a good thing. It's slow, but that's kinda expected. What isn't expected however is the fact that nearly 1 out of 5 albums appear on the device with either missing cover or "unknown artist", even though the artist is clearly included in the ID3 tag.
DoubleTwist even shows the artist in the "artist list" and lists the offending album under the artist's name, but once you select it, you are back to 'unknown artist'. Rebuilding the library doesn't work in that case, either.
USB connection NEVER worked for me, which was quite vexing and nearly prevented me from trying the pay-wall protected wireless sync feature. I understand that this is a samsung galaxy issue, having to do with not being able to use it as media storage device, even when being told to do so. Whatever ...
SD-Card sync is the fastest method and it works ... kinda. You can specify a sync-list for the SD-card and shovel your songs on it (with no option of specifying where to save the files), and the copying process is fast and comfortable.
But once you put the SD-card back into the phone, doubleTwist pretends like nothing happened. No new songs appear in the library, even though it uses the exact same folder as the WiFi sync. And rebuilding the library only yields the songs you synced by WiFi earlier.
And if that wasn't enough, the library of the desktop client corrupts itself on a regular basis, displaying songs twice even with only one instance existing. And there's no built-in feature to rebuild or reread the library ...... no, you can to find the system folder for doubletwist and delete it, so doubletwist thinks it's been freshly installed.
Oh, and you don't like having songs like "ringtone001" or "ambient_forest_001" in your library? Well, bad luck for you, because doubletwist scans your entire device for songs, no matter where they are. And if any game happens to use mp3-files, you'll be able to play them - if you want to or not.
There's a workaround for that ... apparently you can place a a ".nomedia!-file in any folder that shouldn't be scanned ... but honestly, specifying an include/exclude list shoud be in there from the start.
In short (or long, rather) doubleTwist is nearly useless as a wireless syncing tool and only moderately useable as an mp3 player.
Right now I can only encourage people to test it before buying ... and take your time testing!!