I can’t help adding my comments.
I am no adept of Adobe SW due to a total lack of interest to the retribution system of the company.
I have been almost solely using Motion and FCP in a way that slightly differs from what Motion was designed for and probably for what most designers use it for. What I do, I believe, is the a) alternative from Luis and the round tripping work that rowie302 describes for AE and Premiere.
I usually want to integrate relatively long 2D and 3D shots (> 2 or 3 min) designed in Motion to scenes in FCP. The default 10 sec is often not enough for my films. These specific animation scenes a usually unique in my films and are closely merged with cameras shots: lightning, shadows, proportions, perspective, movement, camera angle, etc. As a result I can’t use generators in Motion.
I first create a full length background camera clip in FCP, then I export it to a mp4 file.
Then, I create a scene-specific project in Motion, and I repeat the following round tripping steps: 1) import ReadOnly FCP background clip to Motion 2) develop specific animation clip/s in Motion 3) export the bare animation clip/s to mp4 file/s and import it/them to FCP 4) test the integration back in FCP 6) fix the background clip in FCP if needed and back to 1).
I am aware that this sounds terribly clumsy. In particular, if I need to test with variants of the same background and compare the results.
I wish Motion had somehow a better integration of this basic process with FCP. I don’t find developing titles and transitions thrilling enough for what I want to do. I tried.
Sorry for my lengthy comments.