I have this theory about Family Guy. You see, it's almost too funny. It makes you laugh so much that you forget what was so funny in the first place. And because many of the funny bits are almost completely unrelated to the story (inasmuch as there is a story), it's not like you can remember much by figuring it out from the plot. So all anyone remembers about Family Guy is that it's funny. This apparent amnesia includes the airing date, which during the initial run of Family Guy was all over the place (Sunday, Thursday, Tuesday?). Upon repeated viewings, it becomes easier. Or I'm just a moron who can't remember stuff when he laughs.
American Dad has been less impressive, I think perhaps because it mostly eschews the cutscene/flashback formula of Family Guy. However, I'm an optimist, so I'm willing to give them a chance to flesh it out a bit. I don't think it's as bad as Jeremy Bowers does, but I share his apprehension about Seth McFarlane spreading himself too thin:
I remember when Scott Adams, the author of Dilbert, spread himself too thin with the cartoon and the TV show. I don't have a reference for the quality of the cartoon show without the cartoon, but during the run of the TV show, the quality of the cartoon really took a nose-dive. Most Dilbert daily cartoons before the TV show had effectively two punchlines in the final panel, something that once I noticed really made me respect him, given the constraints of the medium. Other cartoons certainly do it when they can, but Scott Adams pulled it off routinely after his first few years. As he worked on the TV show, the punchline count dropped to an average of one, and it was usually of a lower quality to boot. Now that he's back to just working on the strip, its quality has increased again ...My thought is that McFarlane does indeed drive the whole show (though I'm not sure about American Dad), but I am again optimistic, for some unspecified reason.
... I don't know how much Seth McFarlane is in Family Guy; sometimes the creative guy drives the whole show, sometimes he just sets up a good thing that can live on without him. But if it is the former, I hope that Family Guy doesn't suffer for the involvement in American Dad, or McFarlane may lose big by having two mediocre (and subsequently cancelled) shows, instead of one good one.