For one thing, the film looks great all the way through. The scenery is awesome, be it the Vietnamese landscapes or the geography of the training camp at Parris Island. Douglas Milsome does an excellent cinematography work in "Full Metal Jacket" which was only his second job in that capacity. The soundtrack is minimalistic but it works quite well, never outstaying its presence.
Vincent D'Onofrio and Lee Ermey are absolutely spot on during the first part of the movie. Ermey plays the gunnery sergeant who spends about 95% of his waking hours shouting his guts to the marines under his supervision. D'Onofrio, as Private Pyle, was unlucky enough to be in that platoon in the first place, as he gets picked on by the sergeat and becomes the laughing stock of the platoon. Except no one laughs. Instead they hate him, because everyone pays for whatever he does wrong. Pyle's mind starts to downspiral and from there what happens is anybody's guess.
The second part of the movie has nothing to do whatsoever with the first, except for a couple of linked threads. It all becomes fragmented like short pieces carelessly put together and we find ourselves entrenched in a war zone with a group of marines but not really caring much about them. Not like we cared about Pyle anyway.
"Full Metal Jacket" is excellent for those first 40 minutes which contain one of the best scenes ever shot. The rest is a good, albeit regular Vietnam movie, which doesn't really bring anything new with it and certainly doesn't do it better than other films like "Apocalypse Now" or "The Deer Hunter" before it. It just doesn't seem to be in the "must-see" category as far as Kubrick's catalogue is concerned.