I would not say it is complete trash, but I consider it to be in the upper mediocre area at best. I liked the animations and the OST and Okazaki-kun, but I don't like the character design, the way the "story" is presented and the majority of the characters just pissed me off (or maybe the way they made some of the characters... Kyou e.g. was my secone favorite character at the beginning, but after she went all teary-eyed b/c she didn't realize soon enough that Okazaki was all lovey-dovey with Nagisa, she dropped. A lot.), and in terms of the end I think Air had the far more befitting end. Plus, it was shorter. There were times when Clannad was far too long IMHO.
Honestly, I think there are better love drama series out there... Honey & Clover for instance.