There are a few reasons why Algernon calls his visit to Jack's country estate a success. One of those reasons is Algernon has figured out who it is that Jack is the guardian of. Jack refused, but Algernon persisted and was able to discover it all by himself. In addition to this, he also met this girl and fell in love with her, so that is a major positive to him. The final way this is a success is because it was a successful "Bunbury" for him. He was able to utilize his phantom friend to pull one over his friend to make the trip happen.
