The open source drivers for my ATI card are pretty good. Performance isn't spectacular but it's acceptable, and I've not run into any serious bugs for years, and I have no trouble changing resolution at all.
The performance is acceptable. The problem is I have a weird flickering issue when I run Steam on a multi-monitor setup. The fix is to run a command like this:
Support for the Radeon 7000 series in the open source driver arrived pretty slow. You should check to see what version of the driver is included with your distribution, as it could easily predate the 3d acceleration support.
For older hardware, the open source driver is getting competitive in terms of performance, and of course has the usual benefits of open source drivers.
xrandr --output DFP6 --auto --left-of DFP7 --output DFP6 --scale 1.0001x1.0001
I only had this issue with AMD (not Nvidia).