I think many of us who have heard someone describe crucifixion understand that it was a terrible thing. In Michael Licona’s The Resurrection of Jesus: A New Historiographical Approach he mentions that Cicero commented on crucifixion, so I decided to search for the broader context. It is in Against Verres 2.5.165. He is bemoaning the crucifixion of a Roman citizen named Gavius who cried out over and over that he was a Roman while being lead to his death. One Verres ignored the claim and proceeded anyways.
In his accusation against Verres he says, “You confess that he did cry out that he was a Roman citizen; but that the name of citizenship did not avail with you even as much as to cause the least hesitation in your mind, or even any brief respite from a most cruel and ignominious punishment.”
For Cicero a Roman citizen should never, ever be subject to crucifixion. It is “a most cruel and ignominious punishment”.
On the other hand, a Jewish criminal who may be an enemy of the State has no rights. When Jesus went to die he went to suffer “a most cruel and ignominious punishment”. It was troubling enough of an event that Cicero couldn’t imagine a Roman being allowed to experience it.
the archives near Emmaus 
Sounds like a very modern kind of distinction. According to laws being passed in various states and countries, citizens have human rights, but illegal immigrants, asylum seekers etc don’t have any. We don’t literally crucify them, we just let them deny them even basic care and charity … at least so far.
@Peter: Indeed, it seems that the philosophy of the “civilized” returns in cycles. It wears different clothing, but it the same basic idea.
Cicero certainly reflects (in what was intended to represent a speech which would have been delivered at a trial) a sense that crucifixion was a barbaric and illegal punishment for a Roman citizen. It is however a trial speech intended to persuade an elite jury, and Cicero’s speeches are sometimes somewhat flexible with the facts. Moreover, the fact that Verres, when governor of Sicily, was able to crucify a man claiming Roman citizenship suggests that it wasn’t as unthinkable as Cicero is claiming. Like Josephus, he is a self-publicist, and not necessarily a fully accurate witness.
@Doug: Thanks for the helpful clarification. I guess this shows then there it was more likely to be debatable when someone seen as a Roman citizen was put on a cross than someone who was not. In other words, I don’t imagine Cicero taking the time to argue against crucifying a Jew.