Hot Tamales is one of my favorite places to go in Center City for lunch. They sell great burritos, nachos, tacos, and other Tex Mex fair for reasonable prices. They offer to go or in house seating. It tends to fill up at noon so if you'd like to avoid the lunch rush, try to avoid noon. They service is very good and very quick.
I go to Hot Tamales pretty often since it's the only reasonably priced Mexican food within walking distance of my work. I can't say I enjoy the food very often. At best they're mediocre, and at worst it's just slop. The prices are very low though and the service is fast and friendly. They do offer a nice variety on the menu, unfortunately just about everything across the menu will disappoint. If you're in the area at lunch time and just want some cheap Mexican food, Hot Tamales will do the job, just don't expect an abundance of flavor.
Pros: Cheap and fast