Is it only for motion capture or is it designed for the key framed animator in mind?  Is this a tool I need to learn if I’m a key frame animator?

It is designed for both but by default it is built around editing existing animation or motion capture. 

Software is just a tool, knowing how to animate is the art and you can learn to do that on any software or clay or paper all the software have the same animation tools.  Motionbuilder is nice because you can see the animation in real time so you don’t have to playblast every time you want to see your timing.

I would watch the new demos from siggraph and GDC http://area.autodesk.com/inhouse/videos/autodesk_motionbuilder_gdc_2009_demo on motionbuilder here is another http://www.youtube.com/watch?v=NE73t-eszas&feature=channel there are more there, check out my blog on
http://motionbuildertraining.blogspot.com/ from the The Character Animator Toolkit for Motion Builder masterclass. and try the free trail from autodesk if you want to test it out yoruself after watching the vids.

I have been useing it to do both keyframe and motion editing since 2000 and I have a hard time useing non-realtime applications and software that don’t have animation layers and takes for doing quick tests right in the same file ( now max and maya have animation layers but not always).

Hope that Helps,
Brad

Brad Clark
Co-Founder: Rigging Dojo-Teaching the art and science of character rigging
Author: Inspired 3D Advanced Rigging
The Character Animator Toolkit for Motionbuilder :available now:

Thank you for the reply, I look forward to checking out your videos
Joe