All about the ending (spoilerish)
I'd unfortunately just watched The Devil's Dozen and saw a recommendation for this as a similar but superior tale.
So on I went to unwittingly watch House of 9 for the third time. For the second time, I didn't remember I'd already seen it until the creepy end.
The rest of the film is OK but nothing out of the ordinary. A decent but hardly original premise, with the added drawback that 3 of the 9 cast members are actors I would make a point of avoiding on UK television.
But how I manage to forget it (the end), I don't know. Low key, unexpected and open to interpretation, it should have stuck with me.
7 for the end and 5 for the rest.