I had to laugh the other day. There was an article in our local paper, Por Esto, that kind of firmed up in my mind what I think about them. They usually work the streets in pairs.
It seems that one pair of TP stopped a Candian woman who was driving her golf cart the wrong way on a one way street downtown. One of them told her she could pay her "fine" of 500 pesos directly to him on the spot. Which she gladly did. However, the partner, in his infinite wisdom, gave her a ticket anyway! (can you guess what is coming?)
The tourist was around long enough to actually show up in court. She complained that she had already paid her "fine" and that should be the end of it. The Judge ascertained that she was correct about paying, but that it was not a "fine". The end result was that both officers were fired on the spot. One for soliciting a bribe, the other for not reporting him.
Moral: If you are going to solicite and accept a bribe, don't give the person a ticket anyway!