I dont mind the fact that Facebook wants to follow the "snapchat" style of messaging.
What I do mind is this new user interface. Opinions may vary, but I think it distracts (and detracts) from the previously fairly sleek text messaging part of the app.
While calls and messaging only merit a single tab of real estate, the new image messenger is front and center with an unavoidable shutter button and an entire app for itself. It looms behind the previous iterations interface, a single flick away. I think its a little much.
iPhone 6, ex-alien blue user about Messenger, v100.0