In the ongoing ‘leaks' surrounding the GTX580, it appears that Techpowerup published their review of the card early, but then pulled it (it can still be found via google but the pages have been removed). Reviews of the card are due to go up in a few hours time.
Obviously many forums latched onto this and information from the review has been lifted, including performance graphs. According to the overclockers.co.uk forums the conclusion bullet points read:
Pro
•Substantial performance improvement over GTX 480
•Large reduction in power consumption vs. GTX 480
•Quieter than other cards in this performance class
•Native HDMI output
•Software voltage control
•Support for DirectX 11
•Support for CUDA / PhysX
Con
•Still not as power efficient as AMD's designs
•Current limiter could complicate advanced overclocking
•Still limited to two active display outputs per card
•High price
•DirectX 11 relevance very limited at this time

Above is an image of all the graphs in the review, sorted by a helpful Overclockers forum member. Performance gains look to be much in line with what everyone expected.
KitGuru says: What you were waiting for, or are you hanging on for the 6970 shortly?