How does Apple Motion compare to Adobe After Effects?
After Effects is much more powerful than Motion when you get into advanced stuff like scripting your animations.  After Effects is much more tolerant to large projects, and you can "precomp" different sequences (which is difficult to explain without you actually seeing it; basically allows you to put one timeline inside of another or reuse a specific portion of your project over and over, if that makes any sense).  Apple Motion only allows one timeline per project, you cannot nest sequences inside one another without rendering out a QuickTime movie.

Apple Motion does however boast that it gives you "realtime previews" which plays your animation instantly, whereas After Effects typically requires you to render a RAM preview in order to see your animation in realtime.  Of course this depends on the speed of your computer and the complexity of your project, so this is really only handy for simple projects.

When you want to start doing really custom effects like particle systems and other physics based or true 3D compositing, your options are very limited in Motion.  After Effects has a whole range of plugins available online that will do almost anything you can think of.  The Trapcode plugins are very well-built and very popular.  There are very few plugins for Motion compared to After Effects.

I've used both for years, and Apple Motion seems to crash more often than After Effects, especially as your projects get more complex (just personal experience, maybe this has improved as of version 5).

I actually started animating for the first time in Apple Motion, and I agree that the learning curve isn't quite as steep as After Effects (although that was Motion 2.0 and 3.0, and now they're at version 5.0).  Once I mastered Motion and dove into After Effects, I haven't looked back.  I recently fired up Motion for the first time in a few years and I was struggling to get it to do what I wanted to do, my brain has just been trained to think like After Effects.  Even though at first I was more productive in Motion than After Effects (because After Effects is more complex), I have completely reversed and now I feel like I'm a speed machine in After Effects and Motion is now useless to me.

The bottom line: if you're serious about getting a professional tool to do professional work, save up to get After Effects.  If you're just a hobbyist that wants to dabble in animation, get Apple Motion (now that it's on the Mac App Store the price has come down considerably).

Hope this answers most of your questions!  If you have any other specific ones, let me know and I'll be happy to talk.