Ok, I think I've gathered enough information so now I can do a quick summary.
I've realized that if today you buy an ATI video card, use it under Linux means having to face the following problems:
- Proprietary drivers still quite unstable and lacking some features
- Lack of interest by ATI in the development of linux proprietary drivers which largely share the code with Windows drivers and then depend more than anything else on the vicissitudes of this operating system rather than those of xorg or linux-kernel.
- Support for these drivers unpredictable but in any case, limited to 3, 4 or max 5 years.
- Low speed in resolving bugs and in implementing missing features.
- Impossible to know when the missing features will be added: there is no clear plan.- Need to install an LTS distro or a professional distro so that you can continue to use the no longer supported proprietary drivers for an extended period of time.
(but we all know that installing a distribution of that type will prevent us to get the latest software upgrades and we also know that in Linux world, where growth is constant and software is rapidly implemented with new features (often significant), to be able to upgrade to new releases could be very important).
- At the end of support, need to fall back on the open drivers that although they are more robust than proprietary drivers, they are still in development, lack of features and especially poors with regard to 3D graphics. This probably means a drastic drop in performance.
- Lack of forecasts on the state of the open driver between 2/3 years as in open source is obviously not possible to realize a long-range plan. Therefore currently we cannot know how much the 3D performances of these drivers will be good in future.
- From what I've understood, ATI does not want (at least currently) over-invest in the open project but rather prefers to wait and see how things will develop and how the community will respond. ATI is therefore limited to the following:
- It has made public the specifications of its processors
- It provides direct support to implement the open driver but just enough to make the product usable even if with poor performance.
I came to the conclusion that if currently you buy an ATI video card, you better do so only if the platform you are working on is MS Windows or a dual-boot system.