I recently recovered my lost iPad 2. I cannot afford a new tablet at the moment and I will use my iPad 2 as a daily driver (to browse the web, read books, college lectures, play games and other light applications). My biggest concern is that the iPad might become unusably slow with iOS 9. It runs smoothly with iOS 5 at the moment but most of the applications are not compatible and many features are not functioning (mail etc.).
This Wikipedia Page states that "Although the A5-based iPad 2 is relatively archaic, showing signs of its age in iOS 8, iOS 9 is said to feature performance improvements that may help the aging device function more smoothly, and initial tests suggest that it did not significantly impact available space."
I upgraded three iPad2s last week, and did not notice any degradation in the short time I had for testing. I was making sure that they could be managed by our MDM and that a small set of common apps we use worked.
Do NOT upgrade. 9.3 is a brick on an IPad 2. Stay with version 5 if performance or reliability is an issues. Compatibility with newer apps would be the only reason. I updated and have regretted it ever since. Apple do not give you,the owner of the device the opportunity to backdate should you change your mind AFTER you have updated and are not happy. Not cool Apple.