Performance improvements for the most part are very present. There's a few weird numbers being thrown around, but you've got to remember we're dealing with a beta driver here. Hopefully by the time we get a full-on WHQL one these few issues will be fixed; there seems to be no good reason why we would be seeing less performance with the latest drivers.
Outside of some fantastic performance improvements, there's some really nice new features added to the driver. I would highly recommend that you head on over to the NVIDIA blog hereand read the post as it really explains some of the new features that are seen in the new driver release.
Really, for the most part we've got no idea why you wouldn't make the move over to this driver from the current 197 driver that you're probably already using. If you haven't got a GTX 400 series card, but you're going to pick one up, make sure you grab the latest driver.
Sure, there are a few random issues, but there's nothing too serious. If you're gaming at 1920 x 1200 or 2560 x 1600 the latest driver does a great job of giving us a serious boost in performance which is always appreciated.
It would've been nice to see this driver with the launch of the card, but who are we to complain. It's here now and we look forward to some more GTX 400 cards arriving that are going to be able to make use of the new driver along with the new found performance.