My Radeon DRI3 experience continues to be quite good with the range of GPUs I've tried and different kernel/Mesa combinations from recently. As shown previously, there can be measurable performance boosts for Linux gamers when enabling Direct Rendering Infrastructure 3 on the open-source AMD driver.
For the Nouveau Linux 4.5 testing that I've been working on, I decided to try enabling DRI3 from the xorg.conf when having the latest Mesa 11.2-devel stack from the Padoka PPA on Ubuntu 15.10 with the latest xf86-video-nouveau. Unfortunately, DRI3 there is still not rendering correctly but window borders are black, etc.
That testing has been on Kepler GPUs where I haven't been able to get DRI3 working nicely. If there's some combination with say GLAMOR to get DRI3 working well on Nouveau, feel free to share via the forums. Once going back to DRI2, everything renders appropriately on this open-source NVIDIA Linux driver setup. Hopefully as we move later into 2016 and more Linux distributions are shipping with newer X.Org Server versions and other updated graphics components, hopefully we'll be able to see drivers back to enabling DRI3 by default.