What are the advantages and disadvantages of online games

The biggest pro would be seeing how you stack up comparing with other players, especially if you're into the whole halo or any other shooter thing. Plus, it has proven to be more fun as you get a bigger adrenaline rush with other players as you do by yourself. If you have a headset or anything like that, you can team up with your friends and pwn everyone else before they see it coming!
The cons would be, for the most part, people being idiots online. There are tons of morons online with stupid names, often vulgar, who like to shoot their mouths off more than their actual guns. If you get in a car/vehicle with an idiot, he can really mess you over, as if he crashes the car and ditches you with it, you can die and he'll live on! Also, your teammates can accidentally or purposefully run you over in a warthog, while you are trying to save their hide!
Besides the idiots, there can be server problems, as no one might be on, or there might not be any servers with any more space for you, and then there's the slowdown issue, that comes and goes.