Probably the least interesting of the Covenant arsenal, the plasma rifle is nonetheless a good person to know. Not as drastically powerful as the plasma pistol's charge-up shot, not with the homing goodness of the needler, the plasma rifle is simple, unextraordinary - and quite dangerous.
There is one thing you will do with this: release a rapid stream of very hot plasma. Often. A first-rate weapon for taking on Elites, on easier difficulties all you'll need to do is aim and fire until they drop. Essentially, this is the Covenant version of the assault rifle - only this time, they got it right. There's very few drawbacks, and it's a very handy gun - if hardly unique in any way.
The one thing to keep in mind about the plasma rifle is that, if you fire long enough without giving it time to breathe, it will overheat. You'll have a several-second period in which the thing just sits there, smoking, and you won't be able to fire to save your life. Easy solution: fire in bursts. It may be tempting to just let loose and shoot until it's smoking, as the plasma rifle doesn't have any real recoil to speak of - and therefore will stay accurate - but it's best to stick to bursts; long bursts, extended bursts, sustained bursts, but bursts nonetheless. If you overheat, you'll be stuck meleeing, which isn't always nice. Don't be afraid to grab another one if you do - being the most common weapon of Elites, there's a fair chance you'll be standing on top of another rifle when you overheat.
Speaking of melee, the plasma rifle's speed at it is... relatively fast. Not on par with the plasma pistol, but quicker than most guns, and quite sufficient unless maybe you're having a melee duel or fighting Flood.
The only feature of the p-rifle worth mentioning is its ability to stun. Under a sharp hail of plasma bolts, an enemy will find himself stunnned, unable to move or attack. It's most useful in multiplayer games, where you're more likely to be fighting single opponents - and therefore able to concentrate fire on them - but it can be handy in single-player as well. But beware: don't get so caught up watching them dance that you forget about your heat.
Pros: Goodly rate of fire, good (and sustainable) accuracy, worthwhile melee speed, stun.
Cons: Usual plasma weapon problems, overheating.
Usage: Try using it to increase the effectiveness in whatever your favorite weapon happens to be. It gives a nice boost to nearly anything; e.g. a long-range counterpart to the shotgun, a short-range counterpart to a sniper rifle, a bit of punch for the human pistol. A nice middle-of-the-road choice. If you're not sure what weapon to pack for an area, grab one - it'll probably work fine. Most notable against Elites, and other cyborgs (i.e. multiplayer).
For: Anyone who needs "something more". Love your pistol/sniper rifle/needler/assault rifle, but irritated that they don't always do the job? Augment them with a p-rifle. Also for boring types who like unremarkable but effective weapons.