If you have an older iPad 2 model, you should avoid upgrading to the latest iOS version 9.3 for now. There is an issue where an over-the-air update can stop, and thereafter the device fails activation, rendering it effectively useless. This only seems to affect the iPad 2 model, from around 2011.
The users all say that, when attempting to complete the update, their 2011-era iPads are unable to connect to Apple's activation server.
Hopefully, Apple will address this soon as the update contains several important security patches.
"Even when pressing the 'try again' button on the 'Your iPad could not be activated because the activation server is temporarily unavailable...' message screen, no further network traffic appears," our reader reports.
*Update* Apple has since "updated the update", so that it works as expected.