Just don't waste your money buying those weapon DLC's like I did. You get them pretty early in the game even without the DLC anyway I believe, so they're pointless. As for outfits, can't really help you there, that all comes down to personal taste. I actually bought some of them, but ended up using the standard ones, so yeah, I've wasted money...Grab the Monster DLC's and the 3 extra episode DLC's (Sazh, Snow, Lightning) though, they're worth it.
"Are there any of XIII-2's DLC that's worth buying and not worth it?" How can something be worth it and not worth it at the same time? Is that like a fall and slip injury? Not worth it because you're hurt, but worth it because you can sue?
The only DLC i bought was ROTG which is fairly worth it. The costumes seems nice though but im good with there defaults. --- The strong shall live, the weak shall perish. Survival of the fittest. - Shishio Makoto
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. --- Currently playing Borderlands, Doom, The Elder Scrolls IV: Oblivion, Final Fantasy IX, King's Field, and Secret of Mana.