Phoronix compared NVIDIA's own Linux drivers with the open-source "Noveau" driver across 14 graphics cards, you can check out the results over here. The site concludes Noveau is not too viable for gaming and other demanding workloads, it can be used for basic desktop tasks like office and Internet browsing, but is not recommended for laptops due to the poorer power management.
"Back in September I provided the most comprehensive AMD Radeon
Linux graphics comparison that took 28 graphics cards from all supported
ATI/AMD Radeon product families and tested them under Linux using the
latest Catalyst driver as well as the open-source Mesa/Gallium3D driver.
In this article is a similar comparison on the NVIDIA side as I take
most of the GeForce graphics cards at my disposal and try them under the
NVIDIA binary Linux driver and the community-developed open-source
"Nouveau" driver. Not only is the OpenGL performance looked at for
multiple generations of NVIDIA hardware, but the thermal and power
consumption is compared too. In certain OpenGL workloads, the
open-source Linux driver is now faster than NVIDIA's own driver for
select graphics cards in a fair comparison, but overall the NVIDIA blob
still reigns supreme."