your iPad Air (1?) won't do "proper multitasking". It didn't do split screen before and it won't do with iOS 11.
However, something that i really appreciate is the fact that, while previously, doing the 'slide over' thing to get to the watch app to set a sleep timer while watching a youtube video - the video would stop.
Now it's like: Swipe up, Swipe up -> 'control center' -> hold the timer button, set a timer for 15 minutes -> tap the youtube app again.
The video never stops playing.
That's neat.
I used the betas on my iPad Air - it runs ... fine i guess, but it's not incredibly responsive as it once was.
Will do a fresh install for the full release.