I would like to boil this down a little if I may. I believe that Sondheim is a composer that either speaks to you or does not. I know that some people find him too cerebral and some people prefer to listen to just the music and ignore the lyrics, so they miss much of his appeal.
That being said, there are many fans who are snobs and Sondheim would be the last to approve of that attitude. He does, however, appreciate those who really get what he is trying to do and who relate to a lyric or a song, because that is his goal in working with actors and in attempting to connect to an audience. In that regard, I think he is without match in the industry today.
I do not think he is for everyone, nor are the artists many people have mentioned in this thread. But, you need to respect the man's impact on the business and on composition in general. You don't have to relate to him or like him to acknowledge the art. That is your right. The fact is that any kind of art is subjective and the beauty of it all is that we all relate to different things because we are different people.
Hence there really is no one true answer to this question.
"Always smile at your enemies. It will keep them on their toes"