Just curious to see if there is any consensus on whether this upgrade 
to my Stec System 30 would be worth the $986.00 from Aircraft Spruce.
As near as I can tell, as you pass each succeeding waypoint, it 
enables the auto pilot to intercept and track the course to the next 
waypoint, regardless of the size of the intercept angle. Right?

Angier,

 

Not quite.  Check your GPS ARINC output specs.  On a Garmin 430, a computed turn that requires more than standard rate (perhaps 1/2 standard rate), probably calculated from ground speed, will be left to the pilot with the warning "Steep Turn Ahead."  At approach speeds (120 KIAS), it is a beauty to watch the A/P and GPS + GPSS work to make 90 degree turns by starting a mile from the way point and coming out precisely on the course line and correct MH (typical T shaped GPS approach).  Cruise is most often accomplished "direct to" and no interesting turns are required unless you are racing a closed course with the A/P engaged.

 

Remember that ARINC GPS turn information is converted to turn coordinator commands passed to the A/P.  Just like turning the bug knob on the DG.  That is, the GPSS device allows you to select heading mode on the A/P and then select whether the DG or the GPS is to provide directional control to the A/P.

 

Is it worth it?  You bet.