Depends on what kind of GPU you have.
Intel integrated graphics (e.g. Intel GMA and Intel HD) work perfectly. Intel
actually provides free/libre drivers for these.
Nvidia cards are hit-and-miss, since their free/libre software support
depends on reverse-engineering. Many of them work really well, and some of
them only partly work (e.g. missing 3-D support). Upgrading the kernel to get
a newer version of Nouveau can sometimes help if you have problems.
AMD/ATI graphics are the worst. They generally work, but only in 2-D; 3-D
requires a proprietary firmware blob. These GPUs should be avoided the most.
