The previous version that was available on Nexus 9 was the 5.0.2 that mainly included a few bug fixes from 5.0 version.
Android API level 22, also known as Android 5.1 and 5.1.1, will include quite a few fixes. The update includes the following features:
- Ability to join Wi-Fi networks and control paired Bluetooth devices from quick settings
- Support for multiple SIM cards
- Device protection: if a device is lost or stolen it will remain locked until the owner signs into their Google account, even if the device is reset to factory settings
- High-definition voice calls, available between compatible devices running Android 5.1,
- Improvements to the notification priority system, to more closely replicate the silent mode that was removed in Android 5.0.
Features that are specific to Android 5.1.1 include a memory leak fix, which was introduced with the Android 5.0 Lollipop release and various other bug fixes.
PhoneArena has a list of all possible links to updates on any Nexus you can possibly imagine and even the Nexus 7 2012 got the update. There is still no word on Nexus 6 Android 5.1.1.
The first impression on Nexus 9 is that it all looks the same and it runs fairly similar to performance with Android 5.0.2, nothing out of the ordinary.