While Intel only open-sourced the Broadwell graphics driver changes earlier this month and began pushing the code forward into the Linux kernel, Mesa, libdrm, and xf86-video-intel drivers, the support is moving along nicely.
There's changes that keep going into Mesa and now as of this morning Intel's Broadwell claims OpenGL 3.3 support. The "Gen 8" Broadwell support doing GL 3.3 on Linux puts it on par with current-generation Haswell graphics. While the hardware itself is capable of OpenGL 4.x, core Mesa and the Intel DRI driver right now is limited in its ability to support GL4 / GLSL changes. Hopefully in 2014 we will see OpenGL 4.0 support out of the Intel driver.
Besides advancing the OpenGL support, there's other TODO items left with Broadwell like HiZ -- a performance-enhancing feature -- and numerous other performance optimizations are still to be accomplished. Even for Haswell, at last look the Intel Windows driver is generally still faster.