Corvisiero is definitely a legitimate agency. They have the sales over the years to back up their authenticity, although, as you can see from this thread, in recent times they had some trouble with one of their agents, but it was dealt with. As for whether it's worth attending the conference... it *might* be? This is one of those things you never know about.
I attended a seminar to pitch my book at which different agents, including Marisa Corvisiero herself, were attending and taking pitches from hopeful authors. I totally flamed out of it, though my wife, who had been there for moral support, ending up getting offered representation as an illustrator for picture books when she wasn't even looking for it, so clearly it worked for her, but not for me. If you have the right book, and the pitch is heard by the right person, at the right time, when they have the right hole in their list that they're looking to fill, a conference can be magic. If not, it might not be productive for you, so it's always a roll of the dice.