AMD/ATI have their hearts and/or heads in the right place. They are supported full-featured 3D drivers with documentation and developer time. Most things work already (it's amazing how fast a 3D driver can be developed when the vendor cooperates!), and insofar as a few things don't, it's not because of vendor obstruction but just because of the large amount of complex code and documentation that has to be produced.
On the other hand, Nvidia has *never* helped with the development of the open-source Nouveau drivers. Those only work because of reverse-engineering.
Intel has been cooperating with and funding open-source graphics driver development for the longest time, so their drivers work well for nearly everything. Intel graphics on my laptop work flawlessly with suspend/HDMI/kernel mode-setting, etc. etc. etc.
So yeah, Intel > ATI > Nvidia in terms of practical features, and Intel ~ ATI >> Nvidia in terms of "vendor doing the right thing these days."