I upgraded from iOS7.1.2 to 9.1 yesterday on my iPad 2. I felt forced into to doing this, as Pages/Numbers/Keynote on the Mac is incompatible with Pages/Numbers/Keynote available for iOS7.1.2, and I use all three extensively.
Pros:
• Safari renders much faster.
• Safari crashes much less. And when it does, it just reloads the tab instead of crashing the application.
• Some games run better. Alto's Adventure runs noticeably smoother, ie. more FPS, and with less random loading spikes. Quite surprised by this.
Cons:
• App launch and quit is significantly slower. Both the time itself, but animations also lag quite significantly more. This is the worst thing about the upgrade IMO.
• Safari is much heavier. Takes MUCH longer to launch and be ready, animations lags more. Opening tabs is slower. Selecting the location bar takes more than a second for you to see a response. If you spend most of your time in Safari, don't upgrade from iOS7.
• Most things are marked slower, such as switching between applications, going through Preferences, etc. Curiously, it does happen on and off, as sometimes things responds as fast as I would hope for, and other times there's a clear lag.
Am I satisfied? Well, I would never have updated if Pages/Numbers/Keynote was compatible, let me put it like this. It is not much worse than 7.1.2, but the overall speed bumb here and there makes it clear that there's a difference. Still useable of course, but it is clearly showing its age. I hope 9.2 brings optimizations, you know, the ones Apple promised when they announed iOS9.