Man you missed out. We stayed in San Miguel de Allende (near Guanajuato), and it was absolutely fabulous. Mid-50's at night, low 80's during the day. Best food and dining I've ever had anywhere, period. And at $28 for two people to eat a 5 course meal, you kinda can't go wrong. Our resort hotel room was $35 a night, and from what I can tell the cerveza they brought us to the pool side were gratis. Tack on the $80 for our full day catered guided horseback rides, followed by a couple of hours in the natural hot springs and we had a great time in Central Mexico.
Anything within 100 miles of the border (on either side) is absolute crap. It is expensive and dangerous. Within about 50 miles of the border, ride a tollway/freeway straight to the border, cross, and don't look back.
The best place (in Texas) to cross is at Pharr international bridge. Freeways link it to both sides, and the lines are protected from people trying to sell you stuff. We crossed at about 7 AM into Mexico with no line at all, and came back at 4 PM on a Friday, waiting about 25 minutes to get into the US. I highly, highly recommend the Tamul area. Stay at Adlea Hustexa ($9 a night).