After Ubuntu 13.04 gets released last month, almost two months later, LinuxMint 15 Olivia is released with two default flavour: MATE and Cinnamon. Clem announced it on the Mints webpage and ISOs are now spreading through mirror sites.
There are lots of new features in Olivia:
MDM
Software Sources
Driver Manager
Cinnamon 1.8
MATE 1.6
Software Manager
System Improvements
Artwork Improvements
Upstream Components
For known issues, please refer to the release notes