New York Times had a great article called Darwin’s God last Sunday. It's a lot to read, but absolutely well worth it.
I definitely think that religion and superstition is a byproduct of something that was actually useful for human survival. Stephen Jay Gould used the architectural term "spandrel" for it and I guess that's as good as any. Unfortunately I think religion has a more negative than positive impact in our society, mainly because it can so easily be misused to control followers.
Just read the article, the writing style is excellent and the topic is fascinating!