mental ray for Maya 2015 SP3 is now available for download here.
This service pack ships with mental ray 3.12.1.17
The most important fixes are:
- Progressive rendering in the IPR is more responsive now, especially when finalgather is off.
- Refractions and reflections are enabled for maya_volumefog.
- Deforming animation in Alembic files imported using GPU Cache is fixed for batch rendering.
- Fixes in the mental ray core:
- Unified sampling: fixed possible crash if coverage framebuffer was used in combination with unified sampling.
- Light importance sampling: reduction of noise artifacts when using point lights with non-physical falloff.
- Stabilize flushing mechanism for rendering very large scenes.
- Shader fixes:
- For layering material shaders (MILA), added support for back_scatter LPE pass.
Check out the Maya 2015 SP3 release notes inside the download for the complete list of fixes.
Complete mental ray documentation can be found here.
BTW, some of these problems were reported here and on the beta forum. Thanks for your feedback!
Inside mental ray 
It’s really encouraging to see how frequently and quickly we are getting fixes for mental ray these days. Keep up the great work!
Thank you for making the deep scatter LPE work 🙂
Thanks for all updates but still the problem for coverage with unified sampling for my side I still use in legacy sampling. but is a 1/half?? pixel different when I comp. Thanks for all works guys!