Discovery Science Channel had an interesting and optimistic documentary on a 8 mi diameter impact at the Yucatan and how humanity would fare.

Bottom line, it would kill billions but it would not mean human extinction or even a total loss of civilization. They predicted that the earth would recover in 4-5 years and survival would be possible outside a thousand miles of impact.

They commented that ecologically the earth always seems to do a lot better than scientists predict.

I am sure there would be a lot of dissent in the scientific community. I have already seen one critique that a 3000 foot tsunami could not occur from a land impact. Personally I believe there would be major tsunamis from an impact on the Yucatan of that magnitude.