Right now the only Linux desktop GPU drivers supporting Mir are the open-source Intel / Radeon / Nouveau drivers, but even there it still requires Mir back-end support in Mesa/Gallium3D, which hasn't been merged to master. Back in March on the Mesa mailing list there was the early Mir back-end.
The code back in March was an early patch for supporting Mir as a platform for EGL by Mesa drivers. The patch was seeking comments and not ready for merging at that time.
Since then, Canonical hasn't volleyed any new Mir back-end patches for mainline Mesa integration. It's understandable since the Mir API isn't yet stable and Ubuntu 13.10 isn't close to release yet, but Mesa 9.2 is the release to be shipped in the next Ubuntu Linux release. For those doing any Mesa/Gallium3D driver development or toying with the open-source graphics library, it will be more of a pain dealing with the the out-of-tree support code.
For now the Mesa Mir support is being maintained in a separate Mesa branch that's hosted on GitHub. Looking at the commit history, the Mir Mesa support is still a work in progress and as of last week there's now DMA-BUF DRI buffer support and in May were more changes. Hopefully these changes will be merged soon, but given the code would still need to be reviewed by upstream Mesa developers that so far haven't cared too much about Mir, it's unlikely to happen prior to the Mesa 9.2 branching.
Likewise, for the XMir support, there's out-of-tree X.Org Server patches needed. It's not clear if those patches will be merged in time for X.Org Server 1.15, with the feature freeze needing to be closed soon if hitting its September release target. On the Wayland side, the Mesa support code has been in place since early on while its XWayland code also has yet to be merged to master.
For those wondering about the build steps for these components on Mir, the Mir documentation was recently updated.