It's hard to say much about this movie without giving away important plot points, so I'm only going to make a few very broad comments here.
"Triangle" is one of those films which plays with the viewer's perceptions and expectations, making you think you're watching a certain type of movie and leading you to believe that you know what's about to happen-- only to pull the ol' switcheroo on you. Even when you start to figure out how things work in its strange world, this film manages to not become the movie you're being led to believe it will be. You'll likely figure out a lot of this film's strange logic early on, but even that is misleading, because while you focus on the odd physics of the place where this story takes place, there are still some good old-fashioned plot twists and shocking reveals in store for you. At times "Triangle" feels like it's drawing toward a conclusion when another series of events is actually starting, and its ending feels much more like a beginning than any sort of grand finale-- which is certainly appropriate for the type of story being told. Nothing is as it seems. In fact, it's hard to even define this film's genre; I see other reviewers describing it as science fiction, but that's not really accurate-- science explains nothing which takes place in this film, nor does 'Triangle' speculate about future scientific or cultural trends-- so if I had to try to pigeonhole this film into a specific genre, I might go with "psychological horror," or maybe even "horror fantasy" or "mythic horror." Maybe. None of those categories quite seems to fit.
In short, the writing and acting in "Triangle" are very good, the scenes are beautifully framed and shot, and the editing keeps the story moving without getting in its way. While most of the film's visual effects are convincing, a few are obviously CGI. It's a competent film, and more than anything else, it competently tells the strange story that its producers wanted to tell. This is a smart film which will leave you wanting to watch it again, if for no other reason than to see what you missed the first time around.