Object Spin Fast, Decelerating to a Stop?

The easiest way to do the spin is to right click on the object's X-rotation and Add Parameter Behavior > Logarithmic. Set the start value to something like 7200 and leave the end value at 0.

Another way (with the keyframes) is to open the keyframe editor with the object selected, and for the animated X-Rotation, right click on one of the keyframes and type Command-A to select all, then right click on one of the keyframes and select Interpolation > Bezier. Drag the first control point down towards the bottom rotation value and the last keyframe's control point towards the first keyframes starting frame.

You shouldn't need to manipulate the opacity of the "sides" -- set the project to 3D and arrange the layers in a group "front to back" then spin the entire group (it helps to split each side into separate groups inside a parent group.)

Here's a sample project you can download and take a look at:

http://sight-creations.com/fxexchange/headsOrtails.motn.zip

It's 2 different colored circles with text - one side is "heads" the other "tails". The text and circles are arranged so that as the "coin" spins, you see heads on blue then tails on orange. No changes in opacity. Note the groups are all 3D. I probably put a 1 or 2 pixel Z-position displacement between the two sides just to make sure there was enough separation.

(When people complete "fancy" M5/FPCX projects, is there a place on the forum where they can be shared for feedback and others to see?)

I've only joined in here relatively lately, but to date, haven't really seen any examples of people posting projects for sharing or feedback. I will only post projects in response to direct questions and only enough content to illustrate the topic. The forum at http://www.fcp.co/forum/6-motion-5 invites sharing projects.