First couple of seasons were alright to watch.
Then it got cancelled and should have stayed there. It actually got cancelled twice only to be brought back by fans on the net.
I find most of the characters horrible. Peter is a disgusting man-child. Quagmire is a nasty little man. And don't get me started on the resident paedophile...
I actually find that it is an excuse for the 'writers' to put forth their homophobic, misogonystic, rascist, disabalist and sexist views but oh it'a okay it's meant to be outrageous like that 
And a lot of it is just lazy writing. Copious nods to obscure american pop culture that doesn't translate well over the atlantic. Easy pops at celebrities. And fight scenes/jokes that take up half the show.
I remember one series that seemed to consist of the same storylines that the Simpsons had done before.
Don't get me wrong. I am a huge fan of adult animation. But Family Guy is from so far up Seth MacFarlane's arse it can't help but be full of smug shit.
I do love American Dad though. The characters are actually likeable. And it has actual plots and full storylines. Always a plus.