I've been waiting to see Inside for a few years now. If it's not Romero or Argento, I can wait and I don't have to own the movies directly. But with Inside I waited. I saw some of the other French neo-horrors and liked most of them, but people always told me that Inside was the crowning achievement of French horror. Ok, I'm sure it's the best - Martyrs is one step better according to me, but Inside with it's primitive horror, the concept of motherhood shown both in a good way (protecting your unborn child) and in a bad way (killing everyone just to have a baby) is so powerful.
And as usual with French cinema the ending is ambiguous, because to be honest - either way it ends both happy and unhappy. You'll have to see it.
But the main reason to see it is Béatrice Dalle. She's the scariest character I've seen in a movie since... I don't know when actually. And she's human, and still this demon. - without loosing quality or becoming cartoonish.
Many people have written about Inside, so I'm not gonna waste your time with writing more stuff about it. Just watch it instead, that's the best way of experiencing Inside.