I think lots of sellers don't even know it's there!
When I bought mine (used) the brightness was turned all the way down (off really) and the dealer didn't realize it was there until I told him (AFTER I purchased)
I don't think it is a common feature in All STSs because they're So Many different levels of the car. That is, V6 Basic STSs are NOTHING Like a V8 1SG as far as luxury & features (GM advertised and sold a lot cheap cars to compete). On the High-End versions it's NOT uncommon, just "overall" if you're looking at the entire STS line it might be.
The good news is, when buying used, YOU can buy these loaded-up, 1SG, cars for 3-5K over a Basic car and that buys you the 20K worth of options
I NEVER imagined how GREAT the HUD/ACC were going to be. On a long trip, going somewhere you've never been before, the ACC set to a high speed coupled with the Nav instructions in the HUD is simple WONDERFUL!
Good Luck, wait for the right car to become available - you won't be sorry.....