Ok, now I usually am not one to get car sick. I can travel on mountain roads in the US, no problem. But let me just tell you about the bus ride from HELL!
It started off by buying a ticket from San Cristobal to Oaxaca, a 13 hour overnight bus ride. I thought "overnight, this won't be bad" Well, we left at 10:30 at night. It started off with hard seats. I thought that I could handle that. About 2 hours into the ride, the air conditioning felt like it broke, so it started getting hot. Then we started on the two lane mountain roads: the bus was either going right or left, there was NO straight road. Topping it off by me not being able to see anything, I started getting sick to my stomach. We stopped somewhere in the middle of Mexico for about 45 minutes for a break. That helped a little to get off the bus, but then we had to get back on, and it started all over again. I finally told my friend to give me his bed roll and I said "I don't care who has walked in this aisle, I am laying down!" So I took the bedroll for my pillow, and laid down in the aisle. I was looking up at the ceiling while a little Mexican kid was looking down at me, very curious. I did that for the next 5 hours! I wasn't as sick, but it didn't make it go away. Needless to say, when we got to our destination, I was never so happy to be off a bus in my life.
So, my warning to you is, if you have to take the bus to or from San Cristobal, be prepared for a lot of curves, warm buses and hard seats! Take the plane in if you can!
San Cristobal is a pretty safe town.
As with the rest of Mexico, I wouldn't drink the tap water unless you are accustomed to it, and I wouldn't recommend eating any un-cooked food frrom street vendors.
If you are taking the bus to/from San Cristobal, I would recommend taking some drammamine, because the ride is a little nausiating.
other than the chance of an upset stomach, you are pretty safe.
The Zapatistas have dissapeared to Guatamala or thereabouts, so the only guerrillas you will see are those on the T-shirts of young European backpackers - 'cause terrorists are cool right!... : /
San Cristobal was very famous in 1994, because of the Zapatista guerrilla, now is a secure and pleasant town. And if you´re looking for the guerrilla icons you can found them very easy in the indian markets.