I think the ideal was more along the lines of using missiles to take down missiles, the last article I remember seeing was saying out of 300 attempts they took down 3 ICBM's.

The cost was too high and the technology did not exist to build it. Some improvement in missile guidance came out of the research and other technologies resulted from that research. A few like GPS and some others are still classified.

I don't know...

I'm pretty sure they figured out that it wasn't scientifically feasible. At the time, anyway. I'm not sure if you could do it now.