The trick with NOLA (as with just about any other place) isn't how bad the crime is. There's crime there just like anywhere else. It all depends on how smart you are. I lived down there for years and went to 7 different Mardi Gras. Just be smart. If you park your car outside then don't leave any good shit in it and leave your doors unlocked. If you're walking around stick to the popular areas. In the day you're just fine, and at night if you keep your eyes open you're fine too.
The people that get stabbed or shot are the ones who start acting a fool, start fights, or are doing shit they shouldn't be doing. I clubbed there 3 or 4 nights a week for about 5 years without ever having a single incident.
There's a few rules you need to know though:
1. Don't get involved in a fight, no matter how fucking tough you think you are. There are crazy motherfuckers there who look for shit like that, and if they don't get you the cops will.
2. Don't fuck with the cops. Don't bow up to them, don't talk back to them, and don't try to act all tough in front of your girl. Say "Yes sir, yes sir, yes sir" and walk the fuck away. The cops there wear leather gloves with kevlar plates in the knuckles and have a 'hit first, ask questions at the station' attitude. I've seen a LOT of motherfuckers talk back to a cop, and next thing you know they are tied up on the back of a horse with a broken nose.
3. If you end up on the wrong side of Bourbon St, don't start talking shit about all the gays you see there. The last Mardi Gras I was at 2 sailors ended up there and started talking shit about all the gay guys they met. They found their bodies on Canal street in a dumpster, and even though it was the busiest night of the year and there had to be 10,000 witnesses (literally) not one person saw a damned thing. New Orleans has every type of person, and if you are freaked out by gays, transgenders, transexuals, cross dressers, prostitutes, etc then either don't fucking go or keep your mouth shut.