I am using it only in desktop view because it has a familiar UX/UI to Chat in Facebook. It’s a great way to send out a quick reply. At this point it would be a great feature to merge all messages sent within last 5 minutes into one. Since I use just like chat, I tend to send burst of short messages. The end up being an individual post in the conversation.
But for completely new messages I end up using native messaging to set up the subject. And it’s more comfortable writing longer introductory messages there.
On mobile I find the native experience in combination with Mobile Tab Bar pretty solid. So I hid the quick messages icon there.