Right now, the movies that are worth seeing in 3D are:
Life of Pi
Gravity
Fixed.
I'd say Avatar isn't worth seeing no matter how many dimensions. Hugo was okayish.
Oh. I'm with you in that Avatar and Hugo aren't in the same level as Gravity and Life of Pi.
Hugo annoyed me greatly when watching it because there were so many things about the movie that could have been solved very easily if the kid would just speak up instead of always running away like an idiot. Yes, Avatar was a bit cliche and melodramatic and I've had no interest in seeing it again. That said, it still had some great moments of action and wonder and I was satisfied enough after having seen it at the time.
Despite those flaws, however, I'd still recommend watching those movies to people as long as they saw them in 3D. In both cases, those movies demonstrate a high technical achievement of what can be done with this new medium and how it should be used. Not every movie is going to be perfect in acting, screenplay, directing, cinematography, or technical merits. Most films are good in some regards and lacking in others. In this case, both of those movies are very strong achievements in the technical field and that is why I say they are part of the few movies that 3D is worth it and makes a difference.
As for the movies that do not shoot in 3D but add it afterwards, I've always remembered the Colbert quip when talking about this process for the movie Clash of the Titans: The filmmakers were able to take the 2D images and convert them to 3D while still keeping the characters 1D. Brilliant.