Sazh's DLC let's you play some card games at the casino. The poker game is decent (not great, but good). I play it a lot, so I've gotten my money's worth.
Ye, I was real sceptical at first, but the poker game is actually rather fun once you get the hang of it. : D
I'm more a fan of Chronobind. It's probably one of the easiest way to get casino coins. So, it has its use. So I don't think I wasted my money. But I'm not convinced about the Snow DLC, to be honest. First, I never liked Snow and then... well. It's nice that the second enemy the DLC gives us can drop every unique monster, but I can live without them. Requiem on the other hand... I consider Commander Lightning to be a game breaker and you can get her as early as chapter 2. Though the episode is made of endgame spoilers. But I did enjoy it.
But as said before, the weapons are not worth it. There are plenty of good weapons in the main game. And outfits? Only get those you like. And the Arena Bosses are just that, bonus bosses. If you like the combat in 13-2, you'll enjoy yorself. I bought a few and they were fun.
But in a nutshell, the DLCs are bonusses. If there is a DLC you find interesting, you can purchase it.