Well, I'm totally into the game now. Also, the multiplayer is very much like Horde mode from "Gears of War" with RPG elements thrown in. You earn credits to buy crates full of mystery unlockables (new characters and weapons to use, along with upgrades).
Also, successfully completing levels slowly raises the "Readiness" of the galaxy in your single player game. So there is a compelling reason to play online for a few hours before you try to finish the single player story.
Long live the console war! Buy 'em all! Just don't give Sony any of your Personal info!