When I was in boot, we used irons to qualify, but in the fleet we use ACOGs on the range. However, now even boots are being taught on ACOGs while at MCRD, so they don't even know how to use irons unless they learned somewhere else. I just recently qualified with my scoped M4, and getting expert is just cake if you have any sort of halfway decent shooting form. Like I said though, there are still a surprising amount of people that fail entirely.
I suppose I may also be somewhat biased as I shoot far more than just my military issue stuff and far more frequently than many of them do, being a gun guy and all. Many of these guys only fire a weapon of any sort once or twice a year when they qualify.