A good one, if you treat it as an epilogue to the story. A great adventure with good script. Not a horror anymore, but a great adventure. But yes, i absolutely love this kind of gameplay ("portal gameplay" so most people understand) but it's good when you don't feel like playing a mod to Portal
The ending was even more confusing than in Black Plague and if it was a good surprise there here it just feels like they didin't know what exactly should they do with Phillip. Or maybe there is some other way and i'm missing something?
But if i rate all Penumbra games, they would stand chronologically (including Tech demo) from top to bottom.