Compared to the older drivers like the 260 series that we had running on our GTX 460, we can see some nice little gains. For the newer cards, though, if you're on something like the 263.09 which has been one of the best for the 500 series cards, then there's not much of a change in a lot of scenarios.
Of course, we don't run all the games that NVIDIA mention in the performance improvements list and nor do we run all the configurations. NVIDIA, like AMD aren't going to say there's improvements for something when there's not, and it seems there's a fair bit of focus on higher end configurations like GTX 580 SLI.
Outside of just the performance improvements, there's always a long list of other extras. Starcraft II now has support for "Ambient Occlusion" which helps improves the quality of lighting and shadows to name just a few things.
3D Vision has also got a long list of improvements including a number focused around 3DVisionLive.com, which is a great site to test out the capabilities of your 3D Vision setup. There's also a bunch of game fixes and game profiles added to the driver.
Also mentioned is improved performance for 3-way and 4-way NVIDIA Surround users, more profiles for 3D Vision surround games and just for normal SLI we've got more games added to the profiles along with some enhancements to some games that were already included.
There's also a bunch of stuff under "other" which covers improvements to viewing HD content. For a full list, it's worth checking out the 266.58 driver page here.
Performance improvements only paint a small part of the picture and on a whole you can see some nice extras have been added to the driver. As we also mentioned in the introduction, being a WHQL driver is also an attractive option for users.