Pros: Eat cubic meters of ticks, good alarm systems, monogamous, flock well, pretty
Cons: Free range well but must free range, noisy, like to get into neighbor yards, may sound alarm unnecessarily
"Watchmen" and "Guard dogs" are their #1 purpose on farms, in my experience. Oh, they're pretty birds, lay tasty eggs, taste pretty good themselves, eat cubic football fields bare of ticks and other nasty creepy crawlies, are hilarious to watch, and all kinds of other groovy things can b e said about them... but most people keep guineafowl for their hair-trigger screeching (their best and worst single feature). After all, if you want eggs, get Leghorns; if you want tender, juicy, bountiful meat in artificially low times, get BBWs and Cornish Xs, but if you're looking for a super-inexpensive, self-maintaining and perpetually growing population of tick-eating alarm systems that won't let a hawk, snake, person, predator or anything red get anywhere near your house without their express permission, well, look no further than guineas.
They're faster growing, cheaper, more prolific and easier to maintain than peafowl, while still being great guard animals. Like geese, they're also monogamous (from my experience), but unlike geese, I've never heard of a guinea breaking human long bones, eating all your lawn or pooping in the water dish as soon as it's changed out. Turkeys, while decent flock guardians in some respect, simply don't have the volume when they're threatening something. Turkey threats are just too darn quiet to be proper alarm systems.
My experience has been that they're genuinely entertaining and exceptionally useful animals to have around, and an absolute necessity in woodland settings. I hate ticks, but guineas thrive on the little baznastards and peck 'em up like ambrosia, spilled straight from the cups of Olympus. They aren't the brightest creatures God has ever created, but they're pretty wily in their own right and not prone to predation (pretty well camouflaged, all things considering). They will roost about as high as they can possibly get (which is pretty darn high). Younger guineas sound the alarm unnecessarily more often than adults, and guineas in general may (or may not, depending on individuals) sound off more than geese (who are smarter and don't tend to sound off for stupid reasons). Maternal instinct is spotty in the guinea, and I've heard more of the negative reviews (that they're too stupid to properly raise their own young or even sit on the nest for the required time). Most people I know who raise them put found hoards of stolen guinea eggs in their incubators or let broody chickens/turkeys/geese/ducks/whatever's handy do the job for them. Oftentimes, the guinea will lay eggs into the nests of other birds
They're also difficult to sex (but you can leave them to sort themselves out). Easiest way to sex them is: 1) wait a couple months, 2) listen in on them. Girls make a two-syllable call; boys only have the one one-syllable call. I think the girls sound prettier, but in a way that is either more humorous or haunting, depending on the mood of the person listening.
If I ever get the space to get some, I'll have guineas again. Our guinea cock was hilarious to watch and generally well-behaved (and everyone walking around anytime after dusk knew to avoid our house or that bird would dial it up to 11 and possibly shatter some windows).
Unlike most other common varieties of fowl, guineas as not rapacious jerks. I'm sure someone else will chime in, but in general, I've found them to be pretty reliably well-mannered with other fowl, and especially with their own hordes (and you want a horde because, when disaster strikes at guineas, it strikes really, really hard, and you go from 40 to 3 pretty fast). Guineas are fairly resistant to learning, in my experience, and if they don't hatch with the knowledge of what to do in a given circumstance, they are far less likely to adapt a new strategy or "think on their feet" than other types of fowl. For this reason, it's very helpful to have guineas free range with quick-thinking,flight-ready, adaptable birds like Leghorns. Guineas will generally follow another bird's lead (big group thinkers and conformists like that).