The Washington Post's Brad Plumer recaps a report on border enforcement:
The best outside estimate is that the U.S. government now stops about half of all illegal border-crossings from Mexico. The apprehension rate appears to have increased in recent years as a result of stepped-up border security:
“Based on the best currently available evidence,” the report says, “the apprehension rate along the southwest land border between the ports of entry is likely in the range of 40 to 55 percent.”
That's an improvement, but there's a long ways to go.