I have, too often, done that myself and I almost always regret it. Hai to Diamond stands out in my mind, as does Dog Style. Two manga I put off and put off because the cover art just didn't seem appealing. Yet, the stories were so incredible, I was sad that I hadn't read them sooner.
Never judge a manga by its cover. I thought it was going to be a sport yaoi manga but turned out to be a great yaoi manga which were oneshots ヾ(☆▽☆)