In the finest tradition of sci-fi 3-D movies, the plot is terrible.
The 3-D effects, however, are really very good. I just bought a 3-D viewing system on line and was impressed at how well the home system worked. Of course, one loses the sheer mass of the IMAX screen, but it is pretty cool to see quality 3-D popping out of your home screen (and we just have a standard 19" color TV).
The aliens look great and the roller coaster rides are entertaining, though it would have been nice for the aliens to encounter something else at the amusement park (like a 3-D shooting gallery, for example).