In my usage, battery life has been out standing since watchOS 1 and it only improved with watchOS 2. However, some users reported issues with battery performance so a fix is part of the change log. Fixes for Siri, Sensors, Digital Touch and Software Updates are also a part of this release.
Here is the full watchOS 2.0.1 changelog:
Fixes an issue that could cause software updates to stall Fixes issues that were impacting battery performance Resolves an issue that prevented a managed iPhone from syncing iOS Calendar events to Apple Watch Addresses an issue that could prevent location information from properly updating Fixes an issue that could cause Digital Touch to send from an email address instead of a phone number Addresses an issue that could cause instability when using a Live Photo as a watch face Resolves an issue that allows the sensors to stay on indefinitely when using Siri to measure heart rate
To update your Apple Watch, ensure that it is 50% charged and placed on the charger. Open up the Watch app on your paired iPhone and go to Settings > Software Update. Download and install from there. The update takes a while so ensure that your Internet and WiFi/Bluetooth connectivity between Apple Watch and iPhone does not get disrupted. Our detailed guide for the install process is available here.