The online is amazing you can unlock new guns and customize your favorite guns with it's own camo/scopes which are unlocked by performing kills/headshots with that certain gun. You can call in radars scans, air strikes, and healicopter attacks also by obtaining so many kills without any deaths.
Not to mention the graphics are stunning and from what I saw on the beta ,the servers are great, hardly any lag.
It makes halo 3's online look boring
