Y-You haven't? ;-; Please, go watch it some time (soon x3) It's a classic that everyone should've seen. Clannad is based of a Visual Novel. The Anime is 23 episodes and it basically follows the routes of the VN itself. Pretty good anime with good romance and drama.
First lemme ask you: Are you the type of person that actually feels for the characters and gets into the story when watching something? In general, not only for anime.
Clannad After Story..now that's a whole different story. It's a direct continuation of Clannad and a Master Piece. It isn't the 7th highest fan scored anime of all time for no reason. 24 episodes of slice of life, comedy and drama. If you're even the slightest bit soft-hearted, you're in for one hell of ride. Like I mentioned in the LPW, even years after seeing the anime, I can't listen to some OST without goosebump or moist eyes. But hey opinions differ and everyone reacts in a different way. But it's worth a watch for it entire run. I have the entire series saved on my external drive. I'm meaning to watch it again some time but I can't get myself to do it.
Just give it a try and see what you think~