See I love Family Guy, but just don't get or like American dad or the cleveland show (maybe I have to watch a few eps) but I think Seth should do a good job with Fred and the gang.
American Dad is very good - far more political than Family Guy - but I completely failed to get into the Cleveland Show at all. I don't think Cleveland is a strong enough character to carry a series like that. In contrast, Stan Smith (American Dad) I'd say is a better, funnier character than Peter Griffin. Where family guy can potentially win over American Dad is Brian and Stewie, who are simply amazing creations (though that's not to say American Dad doesn't have other great characters, cos it does).
Getting back to The Flintstones - it'll be interesting to see what they do; how far Seth McFarlane, as a former cartoonist of the programme, takes it into the "21st Century" or whether he remains quite traditionalist.
For the record, I second Gavin's earlier comment. Seth McFarlane is amazing. Can't think of anyone better for such a reboot...