Halo CE: Great all purpose weapon. Can own on almost any map in multiplayer, if the user is skilled. Works at close or medium range. Not very good in campaign.
Halo 3: Good in campaign, because of brutes replacing elites. Good in close range in multiplayer. Someone skilled in H3 AR can easily take down someone using a gravity hammer/energy sword, or shotgun.
Halo Reach: Next to useless in campaign on higher difficulties. Ok in multiplayer, but easily outclassed by plasma rifle.
Halo 2: Necessary in campaign, especially on higher difficulties. Can dominate in multiplayer, especially coupled with a good close range weapon like brute shot or energy sword.
Halo 3: Useful in campaign, but ammo is burned through fast, and there isn't very much of it. In multiplayer BR is a very good all around weapon, and it is highly recommended that one is maintained early in a match.
Halo 2: In campaign, next to useless on its own, but deadly when coupled with plasma pistol/rifle. In multiplayer, very good on any close quarters map, though easily defeated by BR on any map with long sight lines.
Halo 3: In campaign, few and far between, and not as useful as in H2 because of the reduced focus on dual wielding. In multiplayer, a good substitute for the AR on close quarters maps, especially when used with a mauler.
Halo Reach: In campaign, one of your essential tools, vital to playing through the campaign on any difficulty. In multiplayer, a true tool of destruction, though high rate of fire coupled with reticule bloom make it a weapon that takes some practice to use effectively.