The open-source driver stack for the Radeon HD 2000/3000/4000 series is suitable if performance isn't a big concern, if you don't experience any power management issues, if you don't need advanced OpenGL support, and if you don't need any advanced functionality only offered by the Catalyst driver (CrossFire, OverDrive, AA/AF, Catalyst AI, a GUI control panel, etc). It's fine if you're just a conventional Linux desktop users just using the system for office and web browsing needs, potential light gaming, and really don't push your hardware to its limits; but in this case you're likely already using Intel's integrated graphics with their open-source drivers too.
If you're hoping to be running the Linux version of the Source Engine on your Radeon HD 4000 series graphics card, guess again unless you plan to stick around with Ubuntu 12.04 LTS or an older Linux distribution where there will at least be the legacy driver compatibility. It's the same story with the Unigine Engine on open-source, at least not without greatly sacrificing the visual richness of the engines and building Mesa support with patent-encumbered features (S3TC, floating-point textures, etc). For the more demanding open-source games, the Radeon Gallium3D driver performance is still at 50% or less the speed of Catalyst for the Radeon HD 4000 series hardware and it is also that way for the Radeon HD 2000/3000 product families.
This legacy branching of the Radeon HD 2000/3000/4000 series is likely to happen on the Windows side too, but at least there Microsoft Windows has stable interfaces so this driver will continue working for most users and AMD is much more inclined to be releasing legacy driver updates on that side of the OS playground.
Hopefully by the time AMD ends up discontinuing the Radeon HD 5000/6000 series from the Catalyst Linux driver that its open-source driver support will be much better conditioned. On the plus side, dropping this support may lead to some independent Linux developers with HD 2000/3000/4000 hardware to think about supporting the open-source Radeon Linux driver initiative, but for now AMD is about to alienate a lot of Linux desktop customers.