YES THEY ARE! If you hire the wrong DJ they will ruin your wedding. If you have a friend who isn't a professional DJ do your wedding, they will almost certainly ruin the wedding. A DJ is vital to keeping the crowd going, understanding what songs are working or not working, and are key to really making it a professional atmosphere for your wedding. I have seen great DJ's and really bad DJ's, and the difference in how it impacts your wedding is ASTOUNDING.
I'm writing an advice section for our new website and including some information such as this...hope you like it!