I used to be of the opionion that if all out nuclear war begun and every nuke was lobbed off, that humans would be completely dead from the fallout.
However after reading some interesting papers and an internesting link from Winner (you were right Winner!)
I am always right

(at least when it comes to nuclear war

)
showed that even a worst case scenario would only cause widespread devestation and a possibly a maximum of a billion deaths (from blasts, cancer, starvation from contaminated agriculture and failed mechanical infrastructure and local effects) but not extinction.
What are your thoughts on full all out nuclear war? No doubt it would be an immensely destructive event that would take centuries to recover from, but not the end of humanity
Worst case scenario by Winner:
Nuclear war between USA, Britain, France VS Russia, China + separate nuclear conflicts between China and India, India and Pakistan, Israel and Iran+Arabs.
All developed countries would be crippled - the worst hit would be the US, Western Europe, European parts of Russia, Chinese coastal areas. Japan would be almost completely annihilated, as well as Israel and Pakistan. Australia would probably be hit by few warheads too. Middle East would be destroyed (countries like Egypt would probably completely cease to exit).
On the other hand, Latin America and Sub-Saharan Africa wouldn't be hit very severely, they'd suffer from fallout, but not direct nuclear devastation.
I guesstimate that about 2/3 of the US and EU population would die, in China and India, the losses would be even worse. Japan would lose at least 4/5 of its people.
Fallout would be the greatest killer for few weeks following the nuclear exchange. Famine and disease would follow quickly. The human population would decrease for next few years due to radiation sickness, malnutrition, epidemics etc. In the next generation, cancer and mutation would be much more common.
In some areas, the human civilization would collapse completely and revert back to Dark Ages, in other areas, people would manage to keep some level of technical knowledge. Many large nation states would collapse and dissolve (US, Russia, China, India).
Latin America would become the most developed continent, Brazil would gain a superpower status.