I purchased this phone on a rainy day, jailbreaked it the next day and I am much happy of owning an Xperia M today.
Good reliable phone with average 10 hours battery back-up. I am connected 24X7 to 2G GPRS, have voice calls of around 2 hours total a day, chat using Whatsapp n Facebook for Android almost all the time, listen to music for 1 hour a day, watch video for 1 hour, and messaging and other misc. task for 30 minutes.
Altogether, I get a battery back-up of 14 hours, but if the call time exceeds including WiFi usage, battery consumption increases, but never below 10 hours.
Make sure u root ur phone and uninstall Chrome browser and Google+. It eats ur battery. Also, uninstall all high memory apps and replace with light ones’ from Google Play.
ALERT: Never upgrade to Android 4.2. It has a big bug which gives a battery back-up of maximum 7-8 hours only. Maintain it in 4.1 till KitKat arrives.
Thank You!