I regularly travel to the area on business (to Hermosillo) and have gone down to San Carlos regularly.
In terms of safety, I wouldn't worry as long as you're not trying to explore the "seamier" sides of the area (not that I even know of one in San Carlos). In fact, once you're past the border area, you'll find that most Mexicans will be very couteous if you are so with them. It doesn't hurt to know the basic polite phrases in Spanish either.
Now with that said, I ALWAYS get out and watch when they pump gas to ensure you get the amount you are paying for. A Mexican friend of mine assured me that he does too, that it's not just a tourist thing.
I hope to be there next month diving with one of the Arizona shops while I'm down on a business trip.
Hope that this helped.