I've found that FAS is really not helpful 99% of the time.
The only time I really do it is when I'm waiting in line for a bank teller on a cold day. In these cases, you should all do it, since you are not moving.

Without FAS at all, I can routinely get 55 MPG around town.
I'm very happy with this.

There have been a few times when I was tempted to do FAS at highway speeds when I am going down a long mountain grade. But then I learned the the engine MUST spin above 40 miles per hour, even in 'neutral'. Then, in a stroke of genius, I'll admit fueled by GaryG's question about fuel cut ability, I experimented on downgrades at highway speeds, and learned how to get the engine to go 'fuel cut' downhill, at any speed, in D or L 'gear'. I found out how to do this on my own, but Gary's question got me looking into it. So I can honestly thank him for pointing me in that direction.

So for me, at speeds under 40 miles per hour, I don't need FAS to get 55 MPG.
Over the highway, I could use FAS to my advantage, but will never do it since I feel the risk out-weighs the potential gain. Now, with fuel cut mode, I don't need FAS at highway speeds anyway.

To get the engine to cut all fuel ( and still spin ) at speeds over 40, shift to L to get the engine to "rev up". Once the engine revs above normal, all fuel is cut, and the MG1 is spinning the engine with power from the wheels via MG2. You can then shift back to drive, and the RPM will decrease, and the regen will decrease, and your engine will still burn zero fuel while coasting downhill. Even at 75 miles per hour.

Enjoy! Hope that helps all the owners out there!
-John