For me it was quite faithful, you can't expect to put a whole vn almost 60 hours into a few episodes, it's just not possible.
Clannad was a pain to watch, they practically changed the whole plot, yeah and lets forget the movie...
Why clannad is bad adaptation? (not a bad anime but a bad adaptation not an average one like steins;gate, yes i didnt say it was a good one i said it was an avarage one, again adaptation of the vn.)
I think I'm one of the very few people who actually didn't like the anime but then again it's just my opinion and I don't own the truth.