Because I love visual novels and am a huge fan of Key's works, I read the visual novel before watching the anime. I loved it. Although not my favorite, it sparked emotions in me I never knew existed. In most cases (that I've seen at least), when a visual novel is adapted into an anime, it sucks. Therefore, I dove into the show with moderately small expectations, and left extremely happy. Both this and After Story follow the original main plot incredibly well, and the animation is great. They didn't change the fantastic soundtrack too much, which is one of Clannad's best aspects in my opinion.
Overall, Clannad the anime is awesome. If you have not already seen it, do so now along with After Story.