We found that while the condition of the buses varied somewhat (the bus to Teotihuacan was older and shabbier, while the bus back was actually pretty nice), the buses seemed perfectly safe and roadworthy. Many of the other passengers on the route were also foreign tourists, and everyone was quiet and well behaved. The route is all city streets and straight highway--no scary winding cliffside roads! Do be aware that the police may board the bus at stops and record everyone's faces with a video camera. It confused us the first time because the man wasn't in uniform, but don't worry, they're unlikely to bother you.