I lack the expertise to provide a net assessment of chemical weapons’ effectiveness during the Iran-Iraq war, but are some views from the CIA and the ISG report.

This 1988 CIA report says that the weapons were not exactly decisive for Iraq:

But the 2005 ISG report explained how the value that S Hussein’s government placed on CWs:

…CW use helped the Iraqis turn back Iranian human-wave attacks when all other methods failed, buying time for Iraqi forces to regroup and replenish. Iraq again used CW successfully to help crush the popular revolt in 1991.

The report also notes that “Saddam believed Iraqi WMD capabilities had played a central role in the winning of the Iran-Iraq war and were vital to Iraq’s national security strategy.” The report provides a couple of examples:

• Iraq became the first nation to use nerve agent on the battlefield when it used Tabun against Iran in 1984. By the end of the Iran-Iraq war, Iraq had used over 100,000 chemical munitions against Iranian human wave attacks and its own Kurdish population. 
• By 1991, Iraq had amassed a sizeable CW arsenal and hundreds of tons of bulk agent. Iraq had also produced nerve agent warheads for the 650 km al-Husayn missile.