Ah, well to answer your questions:
1. "Is it worth the money?" A: No one can really answer that, it is perhaps a game that you would rent first to see if you like it.
2. "How long is it?" A: Quite short. It's main focus is in multiplayer.
3. "does it have any rpg elements/magic? Or is it just apocolyptic? Melee or just guns?" A: Guns mostly with a bit of Melee and objective based gameplay. It's not a CoD game where you can go solo. As I mentioned before I would rent it first if I was you.
Hope I was some help. I hope to see you online if you get the 360 version