Hani Mansourian fears they will backfire in terms of regime change in Iran. His piece is worth a read. Citing Iraq and Zimbabwe, he notes:
As a result of these devastating effects on the general population, social cohesion and a sense of community greatly deteriorated in both countries, in turn decreasing the likelihood that the massive popular uprising Western governments had hoped would be inspired by these economic sanctions would ever occur...
He fears the same could happen in Iran:
Many years of sanctions coupled with sub-optimal economic policies in Iran have resulted in a weak economy and a fragile middle-class. The latest round of UN, U.S., and EU sanctions on Iran is likely to drive millions into poverty and destitution. As economic opportunities for the growth of a solid middle-class disappear, the young Iranians that have historically been the agents of change in the country will lose their social base. Ironically, then, sanctions may do more to increase the power of the Iranian government and to weaken the domestic opposition movement, to the ostensible detriment of U.S. interests