I downloaded the demo for PS3 and found it was worthless, because nobody was online. So I got it for the 360, and it was okay. It seemed balanced enough that I didn't get completely slaughtered during my first playthrough (I played as a marine). I hate how the demo is multiplayer only. I'll play a little MP, but I'm mostly going to get it for the single player campaigns.
But I just got a Wii, so I won't pick up AvP until I finish Metroid Prime 3, CVA Rebirth, and New Super Mario Bros.