How secure is the border fence? - Not very!
From an American Patrol update: American Border Patrol has performed aerial surveys of the border to show where the fence is - and isn't. If you haven't done so before, take the time to visit American Border Patrol and click on the map of the Tucson Sector in the left. Then click on a yellow dot to see a sector. Click on a number to see a photo. The map hasn't been updated since the DHS stopped building fences, however it is more than 95% correct.
I've seen the border first-hand and can affirm that in many places, the "fence" is nothing more than a few strands of barbed wire. In Naco, a huge steel fence abruptly stops just outside of town, where permeable barbed wire strands take over. In other places (like where Ranger Kris Eggle was murdered in Organ Pipe National Park), so-called vehicle barriers allow illegal aliens carrying loads of drugs to walk across the border unimpeded.