Even in the Marine Corps, many Marines forget them after their initial training. Overall, Marines make sure to be attentive on post and challenge all personnel attempting to enter. The general orders just spell out the reasonable course of action most of the time when on guard duty.
Like most things, it depends. In the USMC infantry the general orders are pretty relevant to daily activity (all the way up to Sgt), so practical application reinforces the learning. I can still rattle off the definition and justifications of deadly force in a single breath, and its been 10 years since I did any interior guard duty. I had to do some scripting in perl 4 years ago, maybe 10% of the day for a year, and I don't remember much of it aside from the shebang at the top and the *.pl file extension.
I too had to memorize these and then once the screaming stopped, I never saw them again.
I think these are a big deal in the Marine Corps and that's it.