We don’t have a specific date set, but generally we are aiming to have it out of beta for the fall semester.
We recently put up a sandbox with some example sites for you to look at, and you can log in to play around with it if you want.
One thing to consider is that the theme work you have done for your existing site won’t carry over to Omeka S, so there’d be significant work to transfer that over.
I know others have raised the question of more granular permissions, but I don’t really know the status of that. It’s not something that we are planning for S, and I don’t think it will be any easier, just different.