As people have pointed out, 3D is becoming less of a gimmick and more something that's just "there" to add to the immersion. Like in Avatar. I guess it's comparable to colour, stereo sound, widescreen, HD... It's not required to enjoy the movie but it's a nice addition.
$20 for a movie ticket does sound ridiculously expensive to me. Out of curiosity, how expensive are they now in the USA? Tickets for 3D movies (IMAX) already are up to $15 over here in the Netherlands.
Edit: I looked up current movie prices at the Pathé site and converted to dollars using Google... The $15 does include tax (which I'm told US$ prices usually don't).