It seems high time to sort the facts from the myth.
First of all, although it has a plausible geographic setting (backwoods Texas) and although its narrator (an uncredited John Laroquette) declares it to be a true story, it is in fact no truer than 'Fargo' (which uses similar tricks to authenticate itself). Claims that it is based on the real-life exploits of serial killer Ed Gein stretch our normal understanding of what 'based on' means.
Secondly, it is remarkably ungraphic. Far more terror is generated from the sound of a chainsaw, or the sound of screaming, than from the sight of flesh being chopped. And even when occasionally acts of violence are shown, they are astonishingly quick, and almost entirely blood free. Hooper is so skilled a director, with so firm a grip on atmosphere and tension, that you leave this film convinced that you have seen far more than he ever actually reveals.
Thirdly, 'The Texas Chainsaw Massacre' is a horror film which does not really scare its viewers, but rather, like 'Audition' and 'My Little Eye', traumatises them, bludgeoning them with scenes of psychological, as much as physical, torment. If you're looking for cheap frights, see a more conventional horror; but if you want to feel as though you've actually been battered by an agonising, visceral experience, this is the film for you.
Fourthly, if you can manage to catch your breath, you may be surprised to find yourself laughing. There is something absurdly recognisable about the cannibalistic family at the centre of this film, despite its inbred backwardness. The grandfather (John Dugan) whose words are uttered for him by everyone else. The son (played by the extraordinary Edwin Neal) who just wants to be an artist (albeit with corpses as his medium) while his brother (Gunnar Hansen) stays at home and does all the work (slaughtering AND cooking). It is shocking, but also darkly funny, to see in this crew of sickos something that resembles any and every family you have ever known.
The film is a low-budget triumph, still powerful in its impact today, and never outdone by any of the successors to the sub-genre ('slash and dash') which it spawned. 'The Texas Chasinsaw Massacre' is cutting-edge horror, and while it may not be to everyone's tastes, it should find its way at some time onto every cinephile's plate.