There was one black spot on our mostly favorable review of the Pebble smartwatch: the music control app just didn't work on some phones. As stated in the review, that's probably got more to do with the phone in question than Pebble's software, since it worked just fine on two separate Nexus devices. Even so, the people at Pebble have been listening, and the latest update to the official app includes a music player selection feature. It will scan your device for music apps and let you choose which one your watch will default to.
After trying out the update on my DROID RAZR M (which, again, would only display track information on the Pebble and not control it) the Music app on the Pebble works great.