My experience with closed source linux drivers is that they're usually very poorly integrated with the rest of the system. The companies usually like to solve everything their own way (tm), rather than using the frameworks all the open drivers use.
When AMD dropped their support in fglrx for my radeon x1300-based GPU in my laptop (yes, they drop support for hardware whenever they feel like it) I had to start using the radeon driver on my ubuntu-machine. Everything has worked much better in the system since the switch. Suddenly I don't get some special AMD catalyst control center-thingie to change resolutions and set up external monitors etc. Instead the normal standardized gnome settings work like a charm. Also the system sets the correct resolution for my screen once, right after the kernel has been loaded (ie. before gdm/X).
If the radeon driver from TFA gets included in ubuntu 11.10 I would definitely give it a shot for my desktop machine, which has a Radeon HD 6870 card. The fglrx support for this card is just terrible. Sure, performance wise the OpenGL works well in games, but the normal X11 2D acceleration is terrible. Here are some annoyances with it:
* Whenever gksudo is activated it throws random garbage on all my monitors for about a second before displaying the password dialog
* Random "holes" in windows at random times, ie. squares where the desktop background suddenly becomes visible instead of the window contents. This won't go away until the window is redrawn.
* OpenGL and XV surfaces are always on top. So if I watch a video and put some window on top of the video surface, the video will be in front of the window regardless of Z-order.
If I could get a driver that plays nice with the rest of the OS, gets regular updates with the rest of the system, and doesn't have weird bugs in its 2D rendering I would gladly sacrifice 50% OpenGL performance. It's not like I utilize the GPU that much anyway.