Our WP-engine hosting is over its allocated quota of 15gb file storage and is being fined for it monthly.
The site itself has 9gb of files, but the duplicated dev environment copy doubles that to 18gb.
It leaves the media files on the host server but copies and links to the ones in S3. It does however give an option to delete the server files, but I have never been brave enough to use it!
That may be a good thing as I just heard from WP-engine that they offer the same media offload service as part of our hosting.
Gravity Forms – we have a few forms that upload to the hosting and a direct bi-pass to S3.