Motion is an amazing compositor and a decent non-linear editor.
In a nutshell, Motion is Apple’s answer to After Effects. While it lacks some of After Effects capabilities (notably built-in 3d rendering) you can get a third-party add-on and still be way ahead. And underlying it is the technology that Shake was built on, so what it does, it does very, very well.
Motion is not something you can just pick up and use (despite Apple providing a few templates you can use pretty much as is), although it’s at least as easy as anything that does what it can do. If you’ve never used a proper non-linear editor or compositing tool before, expect a pretty steep learning curve. In essence, Motion is Photoshop for video, except each layer potentially a video track, key-framed, and festooned with non-destructive “effects” layers (including 2D and 3D transformations).
If you want to edit long form videos, Motion is not the tool you want to use (there’s Final Cut Pro X, if you prefer something cross-platform there’s Hitfilm, and of course there’s always Adobe CS if you’re OK renting software). I tend to work with pretty short stuff so Motion is actually a complete solution for my needs. If I were still putting together short films or chopping up huge amounts of video, I would pick FCPX.
One area where Motion comes up short relative to After Effects in particular is — no surprise — workflow with Adobe products. If you use CS you can import a Photoshop or Illustrator document and get editable layers which you can simply start working with, while still being able to make adjustments locally or round-trip out to Photoshop or Illustrator. But if you’ve got Adobe CS you already have After Effects and Premiere, so either you know what they can do and they still kind of annoy you or you don’t care. Still, it would be great if Motion exposed some kind of API so that Adobe’s competitors (e.g. Affinity’s products or Acorn, say) could round-trip the same way. Oh well, maybe Motion 6.