Paul,
I guess there's one in every crowd, and I'm he. I must disagree with not going to Mexico. It's like anywhere else, whether it be NY, LA, or London, there ARE places you don't go...and border towns are certainly high on the list, including those already mentioned and especially Laredo down in Texas (Brownsville/Matamoros is fine). However as long as you don't linger in the border towns, just clear in and move on, get into the country, you should not have a problem.
I spent March/April in Mexico, Belize, and Guatemala, and over the last four years I've put some 20,000 miles under me in Mexico itself. It's a beautiful country, the food is good and economical, the gas is cheaper than in the US (although the octane levels are not as high as shown on the pump), and if you're a haggler you'll love bargaining with the street vendors. My main gripe is the speedbumps, "topes" in Spanish, there are WAY TOO MANY of them to suit me. Yes, topes, a scarcity of really nice roads, slow trucks, and small town law enforcement types that hit you up for 100 pesos (US$10) for speeding just because you have a gringo tag, even if you weren't speeding, and I wasn't (but that's just typical of many you'll meet down there whose only ambition is to seperate you from your dollars).
I won't go into details here, but if you'd like, email me and I'll be happy to fill you in and suggest a route and offer my own humble opinons.
Steve