I just did a 4 mile run without my phone to test the Series 2 GPS, and color me impressed. Very accurate, def as accurate as my Garmin. I also listened to music with Bluetooth headphones from the watch and used the HRM. The activity was a bit under 30 minutes, and the battery only went down about 10%.
The only thing I'm not crazy about is the phone's exercise app. Good stats on the run but no weekly/monthly stats, and the map of my run, which synced with the phone as soon as I got home, was accurate but didn't show the mile markers like other apps do. Hoping the 3rd party running apps will support the Series 2 GPS soon. Overall though, a very solid running watch.