Well, I think it's a good movie. Possibly even half a great movie.
It certainly starts off sure handed, with Kubrick expertly chronicling the indoctrination and dehumanisation of recruits reshaped into tools of war, but falls badly into pure contrivance during the second act. It also doesn't help that Matthew Modine's 'range' is seemingly limited to two modes: 'mewling' and 'mannequin'.
Kubrick was simply far too late to the subject matter in my opinion. By the time he'd roused himself to give his take on the horrors of war, there really wasn't much noteworthy or original left to say. The hellish chaos of Vietnam, and the ruination it wrought on many a soul, had already been portrayed better on screen several times before.
To be honest, much of Kubrick's work leaves me uninvested in it though. I can admire it from a bravura, technical standpoint...but as a storyteller I find his style overly cold and detached and he repeatedly struggles to imbue his characters with anything approaching genuine emotion.
As a writer myself, story and characterisation will always be the aspects of a movie that leave the most indelible mark on me and the second act of Full Metal Jacket stumbles in both regards.
Just my personal take on it though...and certainly no more valid than anyone else's.