I just played with it for a little while and found it spectacular. I rebuilt a 3D menu for a DVD that took about two hours in After Effects but only 20 minutes in Motion. And the render time is not even 1/5 the AE render.
The 3D is really intuitive and very interactive. It takes a bit of playing with to figure out where certain things are buried. If you drag a group of photos in, they are grouped together as a layer. You can then adjust all the postions in 3D space of each item but also just select the layer to manipulate them all just like parenting in AE. But you have to turn on 3D for the layer which is in a new tab.
There are some nice little things I stumbled on like the fact that when you have turned on record keyframes, all the settings in the Properites tabs turn red to let you know it's recording. No more accidently recording a bunch of position changes.
There is also a new archive function when you save the program where you can apparently collect all the files being used or save it the old way. This will be nice for archiving and moving to a new system.
Overall it seems pretty well done although I haven't gotten too deep into it.