If you are happy with AE, then consider SOME of Motion's strengths:
- REAL TIME design. Watch parameters value changes update immediately while the project is playing. A wonderful feature when the client is sitting in and giving guidance or you don't want to waste too much time while noodling around ideas. It also encourages experimentation.
- Real time particle system. 'Nuff said.
- Behaviors which allow animation that would be difficult (or near-impossible) using traditional keyframing techniques.
- Pre- and post-keyframe control allows repetitive animation with a minimum of keyframes.
- Replicator for easy and quick pattern design.
- Lots of pre-built library items like backgrounds, lower-thirds, etc.
- Roundtrip integration with other apps, including AE.
- Float color space (invaluable for HD and film work as well as more accurate color control).
- Relatively low learning curve.
- Ease of working with animated shapes. Jumping between linear, bezier and b-spline shape control allows a lot of experimenting and control.
I use both for the strengths of each -- it's not a one-or-the-other situation. Motion is an excellent design tool that plays well with others and can save a lot of time. It's also just plain fun to play with. It's not the be-all and end-all of motion graphics tools, and its pretty hefty hardware requirements keep it out of reach of many with older systems. I expect that it will continue to become a powerhouse application that is in the toolkit of more and more motion graphics professionals, and Apple and Adobe will continue to make our jobs easier to create better content with both cooperative and competitive tools. Choice and variety are good when selecting the right tool for the job, and competition keeps everyone moving forward.
Only you decide whether or not this tool will give you an advantage in the type of work you do. Personally, I like to try out any tool that might help me be more creative as well as productive. Use the 30-day demo and see for yourself.