I've been to Tijuana all by myself and I'm a single female. I went during the day and pretty much stayed to the areas deemed "safe" by the advice of San Diego Tourism and US Navy recommended areas. I had a great time! I was serenaded by a Mariachi band and drank some amazing margarita's. I have traveled all over alone so you can take a tour bus if it feels safer. I hope to eventually retire to Mexico if the drug cartel problem is contained. You can live there for real cheap! ¡Viva México!
If you like to eat out (in good places) there's a section in Tijuana called "zona gastronomica" where all the well known restaurants are situated from Argentinian food (cheripan) to Japanese (negai) to Italian (saberios) and Mexican (la querencia), by the way these restaurants offer great drinks too and they are very "chic."
I have been to downtown Tijuana many times. It is oriented to the tourist from the U.S. and from Mexico. There's nightlife on Avenida Revolucion, hundreds of Mexican craft shops, restaurants, and hotels too, with reasonable prices. It's an easy walking place. Playas is outside downtown by some distance and much more restful -- it's accessible by public bus. Tijuana is a nice place to try out visiting the less than first world. There are nice people that often speak both Spanish and English. You will be able to visit the non-tourist places if you speak Spanish. It is a relaxing place, I like it -- I think you will, too. For more info ask at the tourist info booth at the border entrance or the info booth on Revolucion between Third and Fourth. You can get a free tourist map, too.
Mye, San Diego, California
Yeah, it's a great city in Mexico! Here you can visit a lot of places if you love nightlife and cultural life, it's a city that gives a special touch for all tourists. I would invite all tourists from the world to pass a day in Tijuana.