Last night I saw "Titanic" in 3D at the IMAX and, surprisingly, the 3D is actually pretty good.
Why is that surprising? Because "Titanic" wasn't filmed using 3D cameras. Back in 1997 we were perfectly content to watch Leo and Kate legging it around the sinking ship in the usual two dimensions.
Filming a 3D movie using proper 3D cameras means the depth of the extra dimension in every scene is exactly as it should be. If you embark upon a 2D-to-3D conversion, however, you'll need to use software that makes a guess at exactly how deep each object on screen should appear to be.