Some sites are saying it's a rumor, others are saying it's fact. Either way, I think at this point, after a year of hiatus, we can believe that Sulli really is leaving f(X). Her alleged reasoning is that she's interested in focusing on her acting career. While I wish her the best of luck, I think it's a little unfair to fans as well as other members of f(X) that she waited a year just to announce what we already suspected.
It's possible that there's more to the story, though, so I guess we shouldn't throw her under the bus. All we can do is wait for SM to make an official statement.