Unpleasant to watch but ultimately rewarding I was somewhat hesitant to watch this at first - I frown at cinematic gynotorture, and I absolutely hated l'Interieur, a movie often cited alongside this one in reviews. Basically, my stance is: if I'm going to see people (women, especially!) get horribly effed up, there better be a great reason for it. Martyrs succeeds in that respect on multiple levels.
It's a sober and focused work, unambiguous, and mostly free from the kind of self-aggrandizement and indulgence some "extreme" French films fall prey to. The first half or so is prime horror ribeye - sound design, timing, blood, and a certain amount of ambiguity keep the adrenaline pumping and the brain wiggling to figure out what's coming next. The second half...is somewhat different, but that's already saying too much. Yes, the whole thing is torture-y, but it's not excessive to the point of being almost unwatchable (think rape scene in Irreversible), and pound-for-pound MPAA ratings sheet-style, it's not nearly as graphic as, say, Antichrist or even Cannibal Ferox. Nothing but praise for technical execution - acting, makeup, lighting, etc. And most importantly, there's a common (and rather lofty) Idea at work across multiple levels, one you'll ruminate upon after seeing the film much more than upon any single blood-splattered scene.
The only downside is that it's rather unpleasant to watch, which remains a downside even though this film cannot possibly be any other way. The story arc is also a strange roller-coaster, with all the "boo!" moments over and done with way before the movie actually ends.
Definitely worth seeing, if only once.