major sins

They say if you die from smoking, it is suicide, and you go straight to hell. That is why they consider it haram - because it is self-mutilation and "slow suicide" - both of which are strictly forbidden in Islam. The controversy arises when one asks "what if you don't die from smoking?". According to N.American statistics, only 1/2 (or 50% of the) smokers actually die from smoking. The ones who survive.. would it be valid to consider them as sinners?

Re: major sins

Shirk is opposite of 'Touheed'. Shirk word is also used in Qur'an to explain 'touheed'. But the word 'touheed' is not used anywhere is Qur'an. The word which is used in Qur'an is 'Wahid' or 'Ahd' which mean Allah is only one.

Yaqeenan shirk buhat bara zulm hay (Luqman)

Yaqeenan Allah SWT ye baat to maaf nahi karta ke is ke saath shirk kiya jay albatta is ke ilawa (gunnah [sin]) jissay chay maaf kr dy (AlNisaa)

So the biggest unforgiven sin (Akab Al Kabair) is Shirk.

Allah SWT humain is se bachnay ke toufeeq ata farmain Ameen

Prophet SAWS (peace be upon him) said:
"A person will not be able to move on the Day of Judgment until he is asked about ... his body as to what he engaged it in."

Allah says (interpretation of the meaning) in surat al-A'raaf (7:157):
"And He makes good things halaal for them and bad things haraam."

"Do not kill yourself. Allah is Merciful unto you." (Surat al-Nisaa' 4:29)

"Those who cause harm to believing men and women without any reason do a great sin." (Surat al-Ahzaab, 33:58)

"… and do not be extravagant wasters. Those who are extravagant are kinsmen of Satan." (Surat al-Israa' 17:26-27)

Extravagance (in Islam) means spending on something haraam.
Spending money on cigarettes is a waste of resources as well. The Prophet SAWS (peace be upon him) said:

"A person will not be able to move on the Day of Judgment until he is asked about ..... what he owned as to how he spent it."

The ones who survive.. would it be valid to consider them as sinners?
So weather you die bcoz of it or not it is a great Sin.

What if it does not cause your body any harm? Is it still a sin? Because there are thousands of people out there who've been smoking for over 50 years and still live healthy lives.
I don't see the extravegance in it either; it does have some benefits such as stimulation of the digestive tract and mental relaxation.