They are a cut above most recent horror films, and much better than their imitators. They don't shrink from showing the gore, but they are more about ideas and psychological pain. They are whodunnits and psychological thrillers rather than splatter films.
For me the first one was flawed and the second one better, but I can also see the second one is a bit pedestrian compared to the first. Similarly the third one didn't resonate with me as much as the fourth. I've not seen the fifth yet.
They can be hard to follow, especially the third and fourth films which repeatedly refer back to events and characters of the earlier films. That's one of the things I like about them - seeing the same events from different perspectives. They all have complex, non-linear narrative structures, with lots of flashbacks.
I like them. I'm not sure what else to tell you. If you like horror, then you should give them a try.