I am always in DEN. I find that they do a pretty good job of enforcing it - this is especially true for my ABQ flights where about 10 people always just walk up with 3 or 4 and try to board first (I am from there so I can say this without prejudice). Nobody says anything as they push forward, and then the GA rejects them and they sheepishly push their way back through....

The only frustrating thing is sometimes they lump Prem Exec and 1 together, or they will call Prem Exec, and then 15-20 seconds later call 1!

BTW - it is amazing to see how different the mentality is for each destination. Like I said, ABQ people are nervously rushing to get on asap, but the BZN people don't seem to be in any hurry - sometimes the GA has to go tell people to board.