I think 3D is a quick way to increase ticket prices.
I have seen five 3D films in my life. The first, 'The Charge at Feather River' was a very low budget 1950s western whose sole claim to fame was the fact that they threw things at the camera at every opportunity. The second was a documentary about the Valley of the Kings, which I saw as part of the Tutankhamun exhibition at the O2 a couple of years ago. I suspect that it had been converted to 3D because the people seemed to be cardboard cut-outs spaced at different distances from the camera. The third was Disney's 'A Christmas Carol' last year. At least it was actually made in 3D and did create an impressive impression of depth, particularly in the title sequence as the camera swoops over a wintry Dickensian London. The fourth, of course, was 'Avatar', comfortably the best 3D movie ever made, but a long way short of being the best movie in any dimension. The final one was the fourth Shrek film and, having seen it, I don't understand why it was 3D at all. It certainly added nothing.
3D was first tried in the 1950s as an attempt to combat television. It failed - although it did produce one good film, 'House of Wax' (made by a one-eyed director who, ironically, could not see in 3D). They tried again in the 1980s, producing no good films at all, and it bombed again. Now they are trying to force feed it to us. 'Avatar' was good, although not great; ditto 'A Christmas Carol'. I suspect that it will wither and die again in time as people see it for the gimmick that it is and tire of the glasses and the dull on-screen image.
It is perhaps worth noting that another technical gizmo that they tried at the same time is still very much with us and features in the majority of films released now. In the early days, they called it CinemaScope. Now it's called Super-35 when they give it a name at all, and the technology is different - but from the audience's point of view, it's the same thing. It's impressive to look at, doesn't need silly glasses, doesn't dull the image and doesn't cost extra to see. Frankly, I know which I prefer.