Actually the survivability isn't completely known either. There is a good theory that I heard about the K–Pg Extinction which stated that surface temperatures reached about 700 degrees Fahrenheit about 2 to 8 hours after the impact. The theory is that the asteroid threw a ton of earth into the atmosphere, which all then began to fall back to the earth, which created the temperature change almost completely around the world. This explains the death of all insects, the death of all plankton and why all fossils stopped being found for about 10 million years after this occurred.
If this actually happened today, people could survive the impact, the temperature change (being underground previously allowed mammals to survive) , but the overall climate change that would happen for the next 5-10 years, would be very difficult to survive. All plants caught fire previously and that smoke along with the dust from the impact and the volcanic activity that would happen afterwards, would cast a cloud that would make it very difficult for anything to grow for quite a while after it. Not to mention the fact that the fires and lack of plants would severely deplete the oxygen levels around the world. Human survivability would depend on how prepared we were, but also how long the earth's surface is uninhabitable after the impact, if it is longer than 5 years, I don't see how we could survive.