The U. S. Department of State has been issuing traveler alerts since 2008, with wording that becomes stronger with each renewal. Read their July, 2013 travel alert for yourself. If you do decide to go despite the warnings, they encourage U.S. visitors to stay in the well-known tourist areas.
You can check the Department of State website for the latest information, but until things improve, we can't recommend visiting Tijuana on your own. If you want to go, the best thing to do is take a guided tour.