The model is the 15 miles of double fencing south of San Diego. It has been very effective.
And they point to the tunnels. As if there are many and have been highly used. Would be hard to keep a tunnel used by many a secret.
http://www.usborderpatrol.com/Border_Patrol725.htm
Walls are effective, especially if patrolled effectively.
The Wall is opposed by some because they feel it won't work, but most oppose the border fence because it will work a little too well.
Lots of people are making money and getting rich off of border smuggling.
This includes more than a few people who have property located on the border and who are getting payoffs from the cartels to look the other way and allow smugglers and coyotes to use their property as safe crossing zones.
If you find a property owner on the border who is against the wall, it's likely because they are make more money off mordida (bribes) than cattle or cotton