I think it is a huge waste of money that will not solve problems of illegal immigration, drugs and crime, or terrorism.
It won't solve the problem. Not completely, and nothing will. There will always be those who will find a reason (drugs and crime still being lucrative pursuits for some), and a way.
But those are one end of the bell curve. Get down in the middle, and anything to make illegally entering harder becomes a deterrent. Make it hard enough and people who really do want a better life might band together to build one where they are, not just go looking to lay down in greener pastures.
Obviously, some sort of immigration reform is needed, but it has no built in incentives when just going across the fence, hopping over a "normandy barrier", or the like can be done, and with a little evasion and luck, someone can make it to a 'sanctuary'.
IF coupled with stronger enforcement (made possible by reduced inflow), the end of 'sanctuary' areas, the end of welfare benefits, and swift repatriation, I think this has some potential to work.
What I see the barrier as doing is reducing the infiltration to more policeable levels, channeling traffic through smaller corridors, and thus reducing the area to be covered by other assets.