I took them twice. The first time I was relatively happy. Second time was horrible, and I never got on the bus. I purchased a ticket online, once there, I was told the bus was hung up in Nogales. I checked border waiting times in Nogales online, and at 3 am, there was zero waiting time. I hung out about six hours and the bus was supposedly still at the border. I ended up paying a lot in cab fare to go to the airport and pay walk up for a plane ticket. The ticket was supposedly refundable, but I got nonstop runaround. The people at the station said to do it online. I filed the request online, only to be told they had to do it by phone. The phone call didn't happen. I did a request for a credit from the bank. Tufesa fought it for months, with no clear reason why I couldn't be refunded my refundable ticket. I won in the end.
Moral: you never know what you'll get. A slice of the third world in business man at, but the buses are way better than Greyhound.
An hour before heading to the station see where your bus is. It can be delayed by hours or days... So also be flexible.