Pros:
They are great for tick, spider/bug and snake control (to me that is PRICELESS)
If they are allowed to free range they will forage seeds, weeds and bugs off your land for their main food source and consume minimal commercial poultry feeds
They are excellent watchdogs (they will let you and your other livestock know if predators, strangers and neighborhood animals are on your land)
Their antics and quirky personalities are comical at times which makes them fun to own
Their eggs are delicious & nutritious, and if you are up for it Guinea fowl meat is considered a delicacy
When they reproduce, the keets are adorable
Their feathers are beautiful and often used for arts and crafts
Cons:
You usually need more than 2 to have happy Guineas
They can be noisy... and most are, so they may annoy you and especially your neighbors
They tend to roam if they are not trained to stick around, again that may annoy your neighbors
They may want to roost in the trees instead of a coop (and get eaten by predators)
They are wild by nature and not easily trained and tamed like chickens can be
The Hens like to hide their nests rather than lay in nesting boxes in the coop
They have a reputation of not getting along with other poultry, especially during breeding season
I'm sure others will add more