AMD has recently dropped fglrx support for my 3.5-year-old 4850. Meanwhile, Debian testing has updated xorg to version 1.12, which the last-available fglrx apparently does not support. This has resulted in fglrx being dropped* from Debian testing, with Gallium functioning as its replacement. The resulting drop in performance has made XBMC suddenly become near unusable** on my HTPC machine. This would imply that Gallium still has quite a ways to go before it can be considered a true drop-in replacement for its proprietary counterpart.
** Regularly dropped frames in any video, horrible fps (~5 or thereabouts) when merely navigating the normal menus with the Aeon theme.
PS: I personally blame ATI for their ridiculously short support lifetime, and intend to never buy another AMD product in the foreseeable future due to the unnecessary hassle their lack of support has caused. Indeed, I am considering buying a new NVidia card to resolve the issue which AMD has created.