American Dad! is one of those shows that just keeps getting better. The show got off to a rocky start in its first series, definitely being born into its older brother Family Guy's shadow, but since then it has slowly turned into its own show, with its own style of comedy.
Now, here we are with Season 5, and I'm glad this show made it out of the early season danger zone, where animated shows always seem to be canceled. And this set has some of the best parts of the series yet. If you liked the previous seasons, then this should do the trick nicely. If you haven't seen them yet, buy them first, and see if you want to continue.
Overall, this show is not perfect. There are some little irritating character aspects, and they often have trouble filling the episode with a good B plot (although this is something that is brilliantly referenced), but at this stage, I feel that it has surpassed Family Guy in quality and now stands out on its own as one of the better current animated shows.