I agree with post #3 - I think it would be difficult to identify one specific group of people who were particularly affected more than other groups by the invention of writing. As the applications of that revolutionary new idea were developed and spread through all of Mesopotamian society, I can't imagine that any group of citizens was not impacted significantly.
The development of writing allowed the memorialization of laws, business transactions, records of account, including taxes, etc. It demonstrated the superiority of the written word over oral memory which can be faulty, or perhaps deliberately misstated. It was not by coincidence that the first writing was done on clay tablets which hardened, and it was thereby "written in stone," and could not be changed. It was there for all to see.
I believe that ALL segments of society benefited from the development of writing. Hammurabi's Code is a prime example. It applies to both nobles and commoners and provides protection for both. It would prevent the swindling and underhanded dealing that came with oral transactions. It is highly doubtful to me that any one group benefited more than the other.
I would argue that the lives of the common people were the most affected by the development of writing. This may sound strange given that non of them were the ones who could actually read and write. However, the development of writing impacted them because it allowed their governments to be much more organized and efficient. It allowed governments to keep records for tax purposes, for example. This allowed the governments to do a much better job of extracting taxes from the people.
So I would argue that the major impact was on people like this who could be exploited much more efficiently by a government that relied on written records.