Pros-
-They don't really need much attention from you to be happy, just food, a large cage, water and a friend of their own kind, so they can fit people who have fluctuating schedules.
-They are a joy to watch.
-They often don't cause allergies (if they do it is likely the hay not them)
-They sleep when they feel like it, so you don't really have to worry about disturbing their sleep.
-They often don't smell very much (if they do it is likely your fault as their cage needs to be cleaned).
-They are *sometimes* allowed, if you ask, in no-pets rental properties so long as they are kept in their cage.
Cons? (question mark because some people might not consider some of these points cons)
-They need hay 24/7, lots of hay. As such you are likely going to end up with hay outside of the cage. Buying hay in little bags from pet stores costs an arm and a leg so if you can buy in bulk online or better yet find a local farmer it is much cheaper.
-They poop, lots of poop (it's where all that hay goes). The poop should ideally be removed from the cage twice a day.
-They need 1 cup of vegetables per pig per day, in this there needs to be a source of vitamin C (most people use capsicums as they are not likely to cause gas and are high in vitamin C) as they need it in their diet to prevent scurvy (like humans).
-They will get very demanding with their food (Week! Week!), if you feed them as soon as you get up and have their cage in your bedroom you will end up with guinea pig alarm clocks.
-You need to be careful of the pellets you feed, no nuts, seeds, coloured bits or dried fruit. The brands most recommended are KMS and Oxbow (adults only need 1/8 cup of pellets per pig per day). Of those KMS is regarded as better, but is only available for purchase online in the US. If you get a guinea pig prone to bladder stones often you will need to stop giving pellets entirely.
-They need to be housed inside
-They need large cages (much bigger then anything you can find in a petstore)
-They need to be kept in pairs as they are herd animals (normally - there are some exceptions to this rule, normally traumatized and very aggressive individuals) and these pairs should either be the same gender or one/both need to be de-sexed (breeding carries high risks).
-They need exotic specialising vets as small animal vets are not trained in the medical care of guinea pigs.
-Medical care is often very expensive so you need to save up for this ahead of time.
-As prey animals they hide symptoms of illness very well so if you notice something wrong you often need to get them to the vet immediately.
-If they get sick and stop eating you will need to force feed them as they need food constantly moving through their systems to prevent GI stasis and bloat.
-Guinea pigs purchased from pet stores are often sick, mis-sexed, pregnant, mistreated and/or very skittish. It is better to adopt from a shelter or rescue.
-They will never like being picked up.
-They are somewhat addictive - you have been warned
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