This one has been long in the wait. CU1 enables co-existence with older versions of Exchange (2007/2010), which to be honest most have.
Released just now; so not much experience with it so far, but part of the delay has been due to extensive testing, so I imagine it’s all good.
Details on the Exchange team blog:
And of course the download:
This is a 1.3GB download, so I imagine this is a full install.