I will start off by saying that the film is NOT true, but the filmmakers did an extremely great job of making it believable that it was (if you don't believe me, look at the end credits, it will say the 'this film is fictional' jargon).
I'm quite insensitive to horror films; I love Saw, A Nightmare On Elm Street and The Texas Chainsaw Massacre mostly for the depth of characterisation. I didn't think that this film would scare me as much as it did and the reason for that is purely because it is a very psychological film. The legend and backstory behind the Blair Witch is conveyed extremely well and since you are seeing the situations through the eyes of a video camera and three terrified teenagers, it just really gives it that edge. I don't think that there is too much swearing, because in all honesty, it accurately portrays three extremely frightened teenagers which would be doing that in that situation. As the events which happen to them get worse, it shows progressive worsening states of mind, with the female of the group, Heather Donahue, doing the famous confession scene on camera.
I first watched this when I was around 14 years old; I'm now 18 and it is still a film I cannot watch at night. If you have had children that were not particularly bothered by watch Paranormal Activity, The Exorcism of Emily Rose and The Number 23, then they will probably not be frightened by this. It really does depend on the person; many people I know were not one bit terrified by this.
As further warnings (WITH SPOILERS), there are some quite disturbing scenes, for example, hearing children laughter at night, the disappearance of one of the teenagers, a package arriving outside the tent and a freaky finale.