The plasma turret (officially named the "Shade", but I defy you to find someone calling it that), is an interesting piece of equipment. Like the human Warthog's gatling gun, it's a stationary weapon, so you won't be carrying it anywhere (although it's fairly light, and can be thrown quite well by explosions... make of that what you will). However, it's usually placed in tactically strategic positions, so if you get there before anybody else does (e.g. a Covenant), you'll probably have a good angle of view on that battlefield.
Firing powerful bursts of plasma, this is one of the most powerful usable plasma weapons (save for, perhaps, the charged plasma pistol). It fires in triplets, letting loose clusters of three shots every time the trigger is pulled. Capable of fully-automatic fire (and never overheating or running out of ammo), it's a very nice anti-infantry weapon. Its real purpose, though, is for vehicle-fighting, and it does an excellent job of it; the bursts make it easy to hit your target, and it's plenty powerful). It's accurate enough... but unfortunately, its "split-fire" is a disadvantage here. When firing long distances, the segments of fire diverge... and diverge... and diverge... until you're bracketing your target in a nice equilateral triangle, but nowhere near hitting it. Stick to close-range and you'll be happier.
Here's where we get into the depressing part. What keeps the plasma turret from becoming the devastatingly powerful weapon that it ought to?
The thing doesn't MOVE.
Very much like the Warthog's gatling, you're immobile in the turret... but unlike the gat, you're always immobile. Nobody's going to get into here and drive around in it, ever. So you are, quite literally, a sitting duck; and when your enemies see flaming bolts of purple fire pounding them into the ground, you'll have about two seconds until you're caught in a cross-fire from everybody within a mile. You have some protection from the front (in the form of a shield), but from the back and sides, you're wholly open, and consequently, easy to kill. What if you're in a situation where your back's to the wall, and they're only coming from one direction? Well, that's much better, and the turret can be very useful indeed... but you still have to wait until they get fairly close, and by that time they're all over you... and still flanking you. And beware: being stationary, you're a prime target for heavy weapons. Think charged plasma pistol shots. Think tank guns. Think grenades.
Pros: High damage, rapid fire, unlimited ammo, spread-shot firing pattern, frontal protection
Cons: No mobility.
Usage: Use if to thin down a crowd, but be ready in a heartbeat to jump out and run for it. Don't be afraid to just crouch down behind the thing and use it as cover! And if there's vehicles menacing you, give it a whirl.
For: Trigger-happy lazyboys.