D'you want the down-side.
OK, here goes:
(Disclaimer : my DD wanted a pet a couple of years back when she was 9yo. I had GPs when I was 9-22 yo, so we decided on pigs. But DD wanted a boar - which meant TWO boars )
They are manky beasts 
. Yes it's only chopped up veg that comes out of them and it's solid and inoffensive but it does get into every space.They can get good at bladder control (ours do the Dancey Dancey Stamp Stamp which means they want to pee. Ignore at your peril)
But the pooh? They give lift their bums and out it comes.
The hay : ditto. Everywhere
I wake at 3am and think "Are they ok" if they are outside. Too hot. Too cold.
Mine are in at night in winter and will probably go back out in March if it's ok.
They need a load more space than you'd imagine.
Cleaning out their accomodation too (which is my job. DD does loads but I clean) is constant.
When they get ill, it's a worry, they are 0-60 . Our boy was ill for 2 days and even though he saw his Guinea-Vet, he died that evening.
Alot of prevention is the key though.
Safety (escape, predators -cat, foxes, dogs-, things they aren't meant to chew- plastic, cables)
Damp, cold, heat, bullying, draughts
Keeping them active especially when they get a bit middle aged
They do a good line in Judgey. "what's THAT? I don't like celery stalks. FFS"
And they CAN bite. They rarely do. (DH was snapped at by GP3 who doesn't like having his Right Side touched. DH didn't know. He does now . No teeth were sunk in, but the warning was there)
When you go away you'll need to have someone look after them. They need fresh water and pellets daily. We give fresh veg twice a day.
But we adore them. They are lovely.
Between mice, hamsters, degus, chinchillas, rabbits,rats and Guinea-Pigs....I'd pick guineas every time.