I say the cards will run comparatively and competitively with the 5XXX series, being neither significantly worse or significantly better from an all around gaming perspective. Yet the tessellation power will make them stand out. Consider that some cards can last a few GPU generations and still be viable in terms of performance, G80 and some of the 4XXX cards come to mind. These Nvidia cards (as well as the 5XXX) may do just that, but the advantage Nvidia may have, is a stronger future proofing of the tessellation.
Additionally, they are more multi-faceted to be used in markets that ATi does not embrace. While there might be a risk vs. reward cloud hanging over the concept of being involved in such markets, Nvidia can afford to take that risk, and since there is no real competition, the risk is reduced. Similarly Nvidia can and has been able to afford not having a card in direct competition with ATi for several months, giving them time to work on certain aspects that will drive GPU development into the future.
And finally, this generation sees the 1gb memory limit broken. We will no longer be limited to cards that have to share memory with a 1gb cap. How will this translate into performance, who knows yet, but it is a milestone to some degree.