Nvidia drivers in linux are on par with Nvidia windows drivers. For as long as I know, dating back to the 9x.xx driver series Nvidia shares a common code base with all of their drivers. So minus the os specific parts, the goods are the same.
ATI has much better drivers now than back in the day. I've not used them so I can't comment on thier recent usability. The one thing ATI does have going for them is their continuing open source push. They have been playing cach up but with the 8xxx series the current plan is same day open source driver support. I'm pretty sure it's full support to, meaning 3d et al.
Intel's drivers have been great since they started. With their hardware finally starting to come around it looks to be good for mid range things.
The only downside to open source drivers is they are currently limited to 3/3.1 opengl. Intel is pushing hard to bring opengl complience up to current but they got so far behind it's got a lot of way to go. They did however go from 3 to 3.1 in only one development cycle.
