Well, I went to see the Titanic 3D conversion and not only was the conversion great, I'd say it was actually one of the best 3Ds I've ever seen, so a conversion, when done right can look just as good as native 3D. But for it to get this good, it took a whole huge team of people creating animated depth maps and filling in the missing content (when you change perspective you see more behind objects on one side) manually frame by frame. And that's the only way to pull something like that off convincingly.