This is probably going to be the first 3D film a lot of people see, and in that sense, it may well be to 3D cinema what Brothers in Arms was to the CD: the release that everyone will remember later as the one that defined their expectations.
(Or this iteration of 3D may be yet another big flash in the pan - who knows? Though many cinemas have refitted new projectors to cope, I assume those projectors can also do 2D.)
So how was it?
As a film, pretty good, if not up with Pixar's best. But the auditorium was, I think, somewhat stunned by the effects, the sheer 3D-ness, and it's possible that they took a while to warm to it, as there wasn't lot of laughter for the first half of the film. But then again, I think the story does take a while to get going, and that first half is setting the scene for the slam-bang action of the second. The writing isn't deep, but this is meant to be for children as well as adults.
As 3D? Pretty stunning for anyone with full stereo vision. Projecting at 144 fps produces an image stability that makes it easier to watch, and the screen turns from a flat plate to a window into the 3D arena. At one point, I was quite certain a large alien gun was stretching half-way into the cinema.
It's likely that computer-rendered animations will dominate the first 3D releases, since it really only requires a second rendering pass, with slightly different camera angles, and that's just computer time. Indeed, it appears that Toy Story 1 & 2 will be re-released in 3D. But I'm going to have to wait until I've seen a few live-action productions before I can really say how well the technology works: there's an occasional unreality in Monsters versus Aliens that could be coming from the 3D, or could be from the animation.