With the Linux 3.14 kernel that was released over the night, Intel UMS support was deprecated. Intel hasn't maintained their user-space mode-setting support on Linux in about a half-decade with pushing everything these days through kernel-based mode-setting. The Radeon and Nouveau drivers have also become completely dependent upon kernel mode-setting too, with user-space mode-setting these days mostly being left to really old X.Org drivers without a DRM/KMS module. Modern Linux distributions are also beginning to drop support for these old GPUs.
At first there were some performance issues in the Intel KMS code-paths, bugs, and other issues, but those early KMS issues have been worked out for years. Nearly all Intel Linux users are much better off with the KMS code-paths and that's even the only option for recent generations of Intel HD Graphics. However, there's still some very old Intel Linux users out there sticking to UMS. These users are mostly running old Intel 8xx or 9xx series chipsets where sometimes the BIOS goes awry with the Intel KMS driver or there's other system issues if trying to use this new driver, mostly around stability problems.
If you are trying to run Linux on an old Intel i8xx/9xx chipset, you're best off using a few-year-old Linux distribution as the user-space mode-setting code-paths have been completely unmaintained for some time and Intel developers really aren't concerned about this ancient hardware... Heck, it's rare these days to see even Ironlake or Sandy Bridge improvements with the developers now mostly being occupied by Haswell, Broadwell, Bay Trail, and Cherryview.
The next step now that the Linux 3.14 kernel is out there with the deprecated UMS support, Intel's open-source developers are talking of marking the UMS support as "BROKEN" and then dropping the UMS support completely with the Linux 3.16 kernel, per this intel-gfx list post.