Willis is really just a two pitch pitcher. He will typically get off to a fast start to begin his season because of his awekward delivery. Once he goes through a few games and others watch his motion, people are catching on how to hit him. He will be more successful in the future if he is able to come up with at least one more effective pitch.
In other words, in the Fantasy baseball world, if you draft him, deal him when he's at his best unless you see another pitch out of him in the futute.
"Relievers are like volatile stocks. They're the one asset you need to watch closely, and trade for quick profits." --Billy Beane