2009 version. I never saw the 1972 version thus can't really compare the two.
Nutshell: Family goes to vacation home on the lake. Daughter and her friend get kidnapped by recently-escaped-from-jail-psycho and his psycho friends. They go, well, psycho on the gals, stabbin friend to death and shooting the daughter in the back as she swims her escape. Then the happy psycho party end up at the parent's vacation home needing shelter from a storm. Wonder what happens next?
Pretty good as a thriller, pretty darn messed up what goes on. Paced well with okay acting from an ok cast. One might call BS on the whole coincidence of events that come together to make it all possible - but heck, plenty of movies are driven by serendipitous confluences, and plenty of movies are WAY worse about absurd coincidences. Violence and stuff I can deal with, other things, not so much. The rape scene in particular is very hard to watch.
All told, it's an average-to-slightly-better-than thriller. Almost on-par with Funny Games (2007), although Funny Games has a much stronger cast/acting/story and is way more messed up, if you like your Horror/Thrillers to be messed up.
Bonus: Scott from Big Love (Sarah's new hubby) gets a headache.