Now I haven't started the single player campaign, but just on merits of MP, this game is batshitastic and addictive as all hell. I know ziggy and maybe some others are big Halo guys, but this is the way Halo MP should've always been. Badass Nanosuit is just badass, and like all good MP, you can customize it to suit your style of play, whether that be, run and gunner, stealth, or some combination. And the maps (13 in total) are versatile and great as well...not to mention the graphics.
COD4 will probably always be my alltime competitive MP fave, but this could take its place.