One thing I have noticed as I am using iOS 9 beta 1 is this.
It has been a long time and people have requested quick reply features and stuff. Apple has nailed it, in my opinion. I am say, writing a mail, and a Whatsapp message comes in, and I am taken DIRECTLY into Whatsapp, and I can chat for as long as I want, and then just hit "Back to Mail" and I can resume composing mail.
Such an involved and 'native' way to interact is so amazing and I have felt this is Apple's way of implementing quick replies and taking it to another level with making this possible for a lot of other stuff. I am fuelling up and making an entry in Road Trip MPG and a message comes, now I can straight away "go" to messages and type up and then just get back to what I was doing, so natively, and in such a way that feels so connected.