I've spent about 4 weeks in Mexico City total (two two-week trips) and it turned me into a Mexico City evangelist. A lot of people sleep on it, but if you're looking for a city vacation it's hard to beat. Phenomenal food, great museums/art/architecture, super nice people, and the metro system is great and cheap. And everything is super, super cheap.
I've done the Azteca tour and have caught an América game there and had an absolute blast. That place gets rocking. The only louder stadium I've been to is San Paolo. Really, really want to go down for a USMNT.
How did you feel about getting to and from the stadium, assuming you were wearing US gear? I've taken the metro down to Tasqueña and jumped on the light rail to Azteca in the past and have never felt any less safe in DF than in other huge cities, but have never made the trip as an obvious opposing fan, so interested to hear what that was like.