Some people I've spoken with think that Unbreakable was a downturn from M. Night Shyamalan's The Sixth Sense. I'd have to disagree, and I don't think that he started to go downhill until The Village. I stopped watching his movies after that, so I can't speak for the ones that came later.
Anyway, I watched Unbreakable recently when I showed it to my seniors last year. It was the end of the school year, and I was wrapping things up with my lesson on comic books and superheroes. I wanted to show them a film that not only went along with that, but I also wanted them to see something that made them think a little bit. In other words, I didn't just want to show something like Iron Man which many of them have probably already seen. Turns out it was a good choice, and I was able to relate it to a lot of the stuff about archetypes that I had taught them earlier in the year. I also got them to see on their own that it was an allegory for discovering one's true talents and potential.
The people I've spoken with who don't seem to like this movie tend to not be comic book fans. All of the comics fans I know seem to like it quite a bit. While the film is not based on any specific comic book, it's definitely drawing its inspiration from the mythology of comic book superheroes. That's why the ending made perfect sense to guys like me. I remember standing out in the lobby and explaining to some girls what the ending meant, as they were pretty confused. Sure, it might have been that they were just kind of slow. However, once Mr. Glass revealed his true nature, I realized that it only could have ended that way.
Probably what turns off a lot of people is that the film has a somewhat of a slow pace. Personally, I don't think that "slow" is quite the right word for it, as that implies that it's not intentional. Maybe "deliberate" is a better word for it. I have a short attention span, but on repeated viewings, I like how the film takes its time on every shot and doesn't rush to get to any kind of big or explosive moment. I think that's why the big reveal at the end pays off so well. You're so used to not getting any payoffs that you don't see that one coming.
I've mentioned before that if I was doing a "top superhero" movie list, I'd include this film. There would be some other choices that people might find unusual like Robocop and The Mask of Zorro (think about it - what else is Zorro but a 19th Century Mexican Batman?) I'd dare say that on certain days, I'd put this one at the very top of that list.