Release Candidate means that the features have been locked, but some bugs are still being chased. More importantly it’s available for download.
There’s two links you want to checkout:
In the blog post, Michael Atalla is quoted as saying “If Exchange were a standalone business, it would be the 9th largest software company in the world”, that’s BIG, and honestly, it’s a product that I super stocked about being an MVP for.
There’s quite a bit of news in the post including co-existence options going back to Exchange 2003. I suggest though that you have a look at the documentation that’s available as it’s getting better and better every day.
Well done to the Exchange Dev team!