OP moreso than ED. Aside from PV's, my first typical impression of an anime is through their OP. Is it generic or well thought-out? Does it suit the genre? Is it coreographed well? These things kind of matter to me. If it's a quality opening and it fits I'm more apt to watch the anime because I assume that the anime has received the same qualities. It doesn't directly influence my drive to watch, or not watch an anime. But, like I said it is typically a first impression. A good ED is always nice, but never necessary. Often I'll enjoy an ending and then skip it and go straight to the next episode.