Apple is a master at making things seem "perfect" while giving an on stage demo of their latest features.
Haven't downloaded the iOS 9 so I can't be of help, but when I did download iOS 7 beta 1 2 years, the performance and stability was far from what was shown on stage at that year's WWDC.
So just be mindful about app performance and stability as what you experience might greatly vary as to what was shown on stage today.