The Grunts have to be my choice highlight of the game. Consistently hilarious and delightful to watch, they add a welcome touch of humour. Humour may not be to everyone's taste in an FPS, and I'll admit to having reservations when I first heard the comical voices on some preview video, but I was so wrong. The voices and phrases are wonderful, quite apart from the animation. Just check out BCM87 (in which some Grunts take on a Flood attack) and perhaps you'll start to see what I mean.
Personally, I can have a lot of fun just playing hide 'n seek with a Grunt around a rock, watching his movements and listening to his comical footsteps and snorting; and occasionally sneaking up behind and whistling a shot past his ears to make him jump out of his skin. Ok so maybe I'm a big softie, but these grunty fellows are a superb creation.
Standard Grunts wear red or orange life support suits, but I can't say I've ever noticed any difference between them in terms of combat ability. They seem equally soft and stupid. Most carry a plasma pistol or an easily dodged needler, but a few take pot-shots at you with a bazooka-like fuel rod gun that looks a bit too big for the little guys to manage. It's a weapon more usually wielded by the more aggressive black-suited special ops Grunts you meet towards the end of the campaign. Did I just say special ops Grunts? Sounds about as likely as attack-bunnies. Still, that bazooka will hurt, and it can be remarkably accurate when shooting at your Banshee from distance. Death by Grunt is a shameful end for a cyborg, so watch out!
Also watch out for when a fuel rod gun is dropped after the holder is killed, as it explodes a few seconds later. Not a very nice surprise if you've just gone over for a look. On the plus side, the blast can amusingly take out nearby enemies if you kill the owner when they're close to him.
Quick-shot Grunt repellant
Grunts tend to run away after getting hit. You can use this when up against a whole gaggle. Quickly get a shot in on each of them, and they'll probably start running off. At that point you've got time to either take some down permanently, or deal with other more obstinate enemies such as Jackals.
Another tactic to use against a gaggle of Grunts is to quickly flick your fire around the group with the aim of keeping them all somewhat stunned (until dead, ideally), preventing any retaliation. I call this 'Grunt juggling'. It's mainly for use when relatively close, which is when a group can become a lot more aggressive than normal.
Grunts are highly accurate grenade throwers. Your Marines are pretty hopeless at getting out of the way, so it's up to you to stop any throwing. If you're a confident shot, one sneaky but risky tactic is to wait until a Grunt pulls out a grenade, then shoot him so it drops amongst the enemy.
Rather amusingly, Grunts sometimes seem to lose their bearings and lob a grenade in the wrong direction entirely. It's rare but I've seen it quite a few times, notably while battling at the plateau in the rockslide megabattle. Possibly they've been jostled by friendlies or shaken up by a nearby explosion, part way through the throwing animation.
Talking of which, a Grunt is stationary during his throwing animation, which gives you an excellent chance of tagging him with a plasma grenade of your own. I just love doing that - it's so funny. I'll usually shout something like "Right back at ya Grunty!", just to rub it in. Not strictly necessary, but it does give me a warm glow of cyborg satisfaction.
In some situations a Grunt may go grenade crazy, lobbing them at a rate of up to 20 per minute on Legendary (the rate is lower for lower difficulties). See Danger! Falling Grunts for an easily provoked example.