This is not a movie. Not even close. It truly is a treatise on human existence and love. If Nolan starts a religion tomorrow, I'll be the first to convert.

***Spoilers Below***
Edit: Okay, so I saw a lot of hatred and bashing one the net where people said that the ending left a time paradox unexplained. I am not really good at explaining theories but, I found a comment somewhere which does it quite beautifully.

The thing is that Interstellar has real world physics (like general and special relativity along with a perfect depiction of massively rotating black holes) with a healthy dose of science fiction. Nobody knows what's at the centre of a black hole or what a singularity looks like, so Nolan took creative freedom to develop an interesting narrative which though fictional, is entirely possible without violating the already established physical laws. Keep in mind that Kip Thorne was one of the executive producers of the movie.

Another thing worthy of note here is how we interpret higher dimensions and how time appears to a 5D being. Again, Nolan did a really imaginative and beautiful take on what's it like to exist in 5D. It's difficult to grasp all of this at once so, it's only legitimate that there are so many people baffled and positively annoyed with the movie. It's meant to be be confusing. Relativity was counter-intuitive when it was first proposed. If these things were that easy and accessible by the common public, we wouldn't need scientists to trudge through all those difficult equations to discover how things work.

Long story short, here's a possible explanation rationalizing why there's no paradox in the end -
In our 3 dimensional perspective, time is linear. There's beginning and an end or just a straight line that extends on forever. In the so called 5th dimension, time and space is manipulative; it just exists around the 3rd dimension. Like if we drew a line on a paper that represented the 3rd dimension, a circle would be drawn around the line representing the 5th dimension. So basically, while in the fifth dimension, think that everything that ever happens and will happen in the universe occurs at once completely. Beings of the fifth dimension have the capability to manipulate space and time or "time travel" to allow the course of history in the 3rd dimension to exist, therefore their existence as well. From a 3rd dimensional view, yes there is a paradox; but in the fifth dimension, there is no linear story. Just time and space existing all at once and other things that we as human beings could never understand until we evolve.