The preview was first released on July 22, and today the final version is already available for download:
http://www.microsoft.com/en-us/download/details.aspx?id=49161
The final version of Exchange 2016 and the new standard client Outlook 2016 are now available. Exchange 2016 is 1.68 GB in size, making it a little slimmer than the preview with a good 1.69 GB.
There is a detailed article about Exchange 2016 on the Exchange Team Blog:
My download is already running, not very fast, but still. I probably won't be the only one
I'll get started with the installation... In about 5 minutes...