Motion is definitely, beyond any doubt, worth the $50, but it cannot replace FCPX for video editing ease and power. Importing video into Motion and trying to make cuts and other types of direct video edits are quite difficult. About the only easy thing you can do in Motion is "Trim" video ends. When you make a cut, you basically have to copy (or clone) the video and create a new layer, trim the ends to match, etc... It gets very messy, very fast. Even (<choke>) iMovie would be better.
On the other hand, Motion is feature rich in filters and effects to manipulate video. Motion 5 is ingeniously designed to be able create these effects that will work on any video, anywhere, anytime in FCPX using placeholders that represent whatever will exist in the FCPX storyline when you apply the template... not just individual clips (design once, reuse forever). Motion is the special effects/titling designer "arm" of FCPX. Everybody should have it even if *all you do* is open up the templates already provided with FCPX and make a simple customization to make the effect a "default" for the way you use it. Motion is also a Title and Transition designer. Motion is now also a 3D (real) model maker (unofficially - real 3D involves "text" only in Motion to provide paths). Automatic motion tracking can only be handled specifically in Motion so that kind of job has to be exported out of FCPX (shades of round-tripping), but manual tracking can be designed into effects (and I must say, it's a toss up to which is really faster to do -- my vote is for manual tracking... it's just mind-numbingly tedious to deal with, but with the availability of onscreen controls, it can be quick.)
Whereas you can create 3D Text in FCPX, you do not have access to any of the parameters that will allow you to make custom animations with the text. Creating the same title in Motion and publishing those parameters will make those customizations in FCPX available to you.
The learning curve for Motion is much longer than for FCPX. There are tons of free tutorials around to get you started quickly though. And, you can ask about anything here.
If you're serious about editing video, you have to have FCPX. If you're serious about creating your own customized effects, titles and transitions, you really should have Motion as well, but you can live without it. There are plenty of free and paid effects available that can be added to FCPX without requiring Motion—but if you were asking me directly, I'd try to talk you into it as an add-on for fcpx.
