The performance-finicky Olin Coles at Benchmark Reviews recently updated from NVidia Forceware version 175.16 to the shiny new 175.19 drivers.
What did he get for his trouble? A staggering 40% decrease in performance. Minimum.
As I have previously mentioned in this article, it's my belief that drivers should improve stability and performance with each new revision. Now, one could argue (if they had the data) that NVIDIA enhanced stabililty at the expense of performance. That someone would probably be playing Devils advocate, because I can't believe any reasonable person would decide that very good stability raised to extremely good should result in very good performance dropping to fair. This kind of argument also doens't explain why 3dMark06 would see a 55% reduction in performance between driver versions.
Just a reminder that even in drivers, newer is not always better. To their credit, NVIDIA's already pulled the drivers... although you have to wonder how drivers with that sort of performance drop made it live in the first place.
NVidia Forceware 175.16 vs 175.19 [Benchmark Reviews]