I cannot make a qualified statement on what is "good" and what is "bad". I can only have an opinion as to whether or not amateur adventure games are pleasing to myself and myself alone. That being said, I consider the majority of ALL games to be "bad", and it doesn't matter if they're made by a huge team in a studio or by one person in a basement. However, I do like quite a handful of games of both categories as well.
If we are using the true definition of "fangame", which is a game based on an already-existing franchise rather than one with an original premise and characters, then I can honestly say that I've never played one that I liked. I've never read any fan fiction that I've liked, either.
Fan art, however, is a different story.