Does it really work?  If I use it will I really never have to trim anymore?

Yes. It's magic.

I use a full length neck bushing sizing die.  Brass doesn't grow after initial sizing (if it is needed).  THis provides less working of the neck as compared to ball or mandrel inside neck sizing.  Brass lasts longer.
Consider, when sizing with a ball or mandrel the neck ID is first sized down to less than the size of the ball or mandrel . Then as the ball or mandrel is extracted throuhg the neck, the neck ID is expanded to the size of the ball or mandrel.  Compression and expansion for each sizing operation.  With a bushing die the neck is only sized down from the fired size once per sizing operation.

Who makes the X, RCBS?

The X-die works as advertised.

Don't see a 6.8 X die

ETA:

Called - they don't make one

Yes.