I never use the new inbox, it's very cumbersome. It creates multiple threads for channel messages and sms, creating confusion.
Legacy inbox is very intuitive and very convenient. It should be the default messaging system.
Ownerrez mentions that legacy inbox will be removed soon. That will be a big mistake, please keep it.
I too find the new inbox to be too cumbersome. Bad design. I don't know how other services handle a unified inbox, but I always go to the original.
A related change is that texts of sent messages stopped being selectable in order to copy it, that to select them, it's required to click on the text fields, which sends you to another page where you can select it. Makes no sense.
