I'm not sure if I agree with the previous postings. I very recently came back from Mexico City. While in line for customs, there are numerous prominent signs instructing you how to fill out the forms, complete with little arrows and x's showing you where to sign.

These signs are almost uniformily ignored. Apparently the passengers feel that the instructions and the forms are for everone else but could not possibly involve them. The passengers express total suprise when they finally get to an agent and then have to fill in the information. Instead of getting out of line, they get an oh gee golly wiz attitude, I have to fill this out and stand in front of the agent while they leisurely fill out the form and then reread it, while everyone else stands in line waiting and waiting and waiting. Did I mention that we were waiting?

I hope I don't sound bitter

When I was there, they had all of two agents working, or at least assigned to the customs booths. There were many many more people assigned to get you into the duty free stores after you clear customs than there were custom agents.

We were one of the first off the plane, and it took f-o-r-e-v-e-r!
I can not imagine what the time factor was for those who were last off the plane. I would plan for LOTS of time.