NOLA isn't as bad as people make it seem these days, you just have to be smart about it, else bad things can happen. I was born an raised there, and didn't have many problems myself, just gotta have the "street smarts."
On the topic of messengers, I never saw one before Katrina, and I have never seen one after. As someone said before, the streets are terrible both in terms of potholes and in terms of layout (it isn't a grid system and there are no bike lanes). The heat is also a serious issue, I could easily see someone getting significantly dehydrated very quickly just riding around down town.