Using the success of the Patriot missle system in Iraq against SSMs (admittedly only semi-ballistic), the opponents of SDI don't have much credibility with me.
As SDI technology improves and matures as a result of increased research and testing, eventually ICBMs will be about as useful militarily as massed cavalry charges against machine guns.
The primary argument against SDI has always been mainly a bit of ideological hysteria from the Progressive Left around the world... hating and fearing the power and freedom that the American economic and political system have produced.
SDI is just a matter of solving a complex technological problem, something the American system does better than any other system... and I would much rather have such a defensive system in the hands of America than any other country.
Who knows, maybe with a deployed, successful SDI in place Pakistan, Iran, India, N. Korea etc., can put the resources they waste now on nuclear weapons and delivery systems into more productive uses for the benefit of their own citizens.