QUESTION: WHEN LORD KRISHNA SAYS THAT HE IS THE DOER AND NOTHING IS IN THE HAND OF MAN, ARE WE RESPONSIBLE FOR OUR ACTIONS, WHETHER GOOD OR BAD?

Many people interpret The Holy Book Gita as per their thinking and understanding.

My point of view and understanding:-
God or say Krishna is the Creator of Life but he is not created by anyone. He created Life (humans) and allows humans to perform Karma. Now what I want to say that, "HE IS THE DOER" means by providing Soul and Energy to a Body (human), he has done his doing, means by virtue of Krishna we have the power, we have the life, we are alive, so we are doing our daily tasks. (However, as he is god, his doings are not accumulated in any account as in case of humans).

Importance of Karma Yoni

"NOTHING IS IN THE HAND OF MAN" means that, without Krishna's or say God's consent no one can get rid off the system of life and death, nobody can ignore the process of karma, no one can get more or less w.r.t. their karma.

In short, Krishna created us so he is the doer to take us in this life system, it is not in hand of a man. But while our stay at earth, we can only perform our karma or deeds, which in turn gives us the results.

QUESTION: WHY DO SOME PEOPLE CALL SRI KRISHNA "SUPREME PERSONALITY OF GODHEAD" WHILE NO VEDIC SCRIPTURE DESCRIBES HIM SO?

Sometimes I feel, as Dwapar Yuga i.e. period in which Krishna stayed at earth, was only 5000 years old, we have found many things which proves the existence of him like submerged Dwarka Nagari (in Gujurat), Mathura & Vrindawan (in Uttar Pradesh) etc.

However, we are unable to found much more evidence of other Gods which are millions years old. So it might be a reason, people trust the existence of Krishna is more than other deities, so they used to call that "Krishna is the supreme personality of godhead".

BUT, LORD SHIVA IS THE SUPREME DEITY OF ALL THE GOD HEADS AND THE SAME HAS BEEN WRITTEN IN LING PURAN.

Refer:-

http://basicsofhumanlife.blogspot.in/2014/03/birth-of-universe.html

http://basicsofhumanlife.blogspot.in/2014/03/role-of-lord-shiva-in-creation-of.html

QUESTION: WHAT ARE THE BENEFITS OF WORSHIPING GOD KRISHNA?

Worshiping any God will benefit us, but why Lord Krishna in this KALYUGA.

Actually, our holy books said that, when you worship some god, you will enlightened by that god's characteristics, like you will think like the way that god thinks, in short you are following him and you try to think same ways.

Just for an example, Lord Rama, was a very noble & decent god, he was full with mercy, he was very eager to forgive.

But as we know, Krishna Ji, was in DWAPAR YUG, after it, KALYUG was going to start. He was not like Lord Rama, He was on to the path and strict with the laws, whatever u do, u have to suffer, either good or bad. He shows mercy but with a certain low level. 

"HE WAS VERY FOND OF KARMA & THE SAME HE TOLD IN GITA"

Benefits are like, In this Kalyuga period, we have to tackle all types or person, we have to be good with the good, but have to beware from the bad. I personally feel that, Krishna's message was very much applicable in modern time (Kalyuga). If we worship Krishna Ji regularly with devotion, we will think and behave like him only which will help us.

Gita updesh by Lord Krishna to Arjuna