This release is a lot more polished than the developers version that was made available last month, with iOS 9 beta 3 being sent to the devs just yesterday. The latest version of the OS has quite a number of differences compared to the previous one, with vast improvements on Siri, split-screen multitasking and other less-obvious performance upgrades.
If you are keen on trying it out for yourself you can get into the program here, just be aware that it is still in development so you will obviously come across some bugs. So, do your backups before attempting it just to be safe.
To find out more about what iOS 9 has in store, you can check out our compilation post here.