I'm of two minds with this book. 
 My first mind revolves all around SF and SF concepts and good plots and great characters and deeper feels and plainly fun writing. 
 My second mind is content to have a novel that's mostly just about the immigrant condition and have a mostly realistic if slightly too regular action revolving around a strained relationship between two rather different people forced together by circumstances. 
 The second mind considers this novel to be rather literary and super-grounded in everyday and everyman concepts, attempting to be universal while barely touching upon anything extraordinary. This is true despite the fact that an apocalypse has come and portals to other places start turning normal doors into a random exodus on the Earth. This concept is barely explored. It's just a dirty handwavium and is used as a very convenient plot device. The fact that war devastates everything and all the normal lives are thrown into upheaval is just a setting, not something to have thoughts about. 
 This is fine if all we want is a character novel that makes light work of everyday chaos and instead tries to show us that normal relationships will still try to work (or fail) regardless of setting. We aren't required to have any kind of stability to live. Our two main characters here are caught in the normal struggles of very different people trying to make a go, drift apart, or otherwise dance the somewhat sad and complicated dance that is all relationships. Theirs is not a happy or exciting relationship, but it is a complicated one. The author is striving for realism and he gets realism. 
 The other reason this book might attract readers is the fact it's distinctly Muslim. 
 On the other hand, my first brain is rather disappointed. I wanted SF and this is about as mild as it comes. A brief mention that doors become doors elsewhere is all we get. The rest is just mild survival stuff with mostly running into nice people who give these kids a place to stay once they finally leave their homes. Later on, it's just the immigrant condition of making a living, falling apart, and later, wondering what happened. 
 As an SF it's almost nothing. At least in other "literary" works I could mention like DFW or Rand or Atwood, a lot more attention is given to making the SF interesting and thought-provoking. 
 This one was just a silly plot device that is below the standards of 1930's bad pulp. Sure, the literary realism side is nice and readable if that's what you want, but as an SF? Look elsewhere. It can't even be considered a post-apocalyptic. 
 I suppose I'm not really a fan of the super-super mild brand of literary fiction that borrows a single super-simple SF concept to tout itself as being on the forefront of the genre. It cheapens the really and truly excellent authors who have done amazing things in the field in so many brilliant ways. 
 If he's trying to fool the literary crowd into buying into the SF market, then fine, but that market needs to realize this kind of novel is baby/baby lite SF that's more like a shadow of a shadow of what it could be. I suppose it might be best to just call it literary and strike off the idea that it might be SF. 
 Ian M. Banks, this is not.