Fairly accurate. R. Lee Ermey was an actual marines boot camp instructor. He "wrote" all his own lines. From imdb:

According to director John Boorman, Stanley Kubrick wanted to cast Bill McKinney in the role of Gunnery Sgt. Hartman. However, Kubrick was so unsettled after viewing McKinney's performance in Deliverance that he declined to meet with him, saying he was simply too frightened at the idea of being in McKinney's presence. Kubrick then hired Tim Colceri to play Hartman. Colceri never got to play the role, as former US Marine Corps Drill Instructor R. Lee Ermey, consultant for the Marine Corps boot camp portion of the film, performed a demonstration on videotape in which he yelled obscene insults and abuse for 15 minutes without stopping, repeating himself or even flinching - despite being continuously pelted with tennis balls and oranges. Stanley Kubrick was so impressed that he cast Ermey as Hartman. Colceri was bitter but accepted Kubrick's consolation prize of a small role as a helicopter door-gunner.

These days they treated us a bit less harsh and much more politically correct. But it still is the same in many ways. Now there's women as well. I'd say it's about as accurate as any other film version I've seen. But it was much more grim in the movie. Of course, I hadn't been drafted and had volunteered in peace time, but we goofed off a lot more than was shown in the movie. There were lot's of harmless pranks, and little of the "soap party" parts.