If the Chronicles of Narnia are C.S. Lewis' attempt (and a wonderful one) to write Christian children's fables, then this trilogy seems to be his attempt to write Christian science fiction.
Although I am not Christian myself, this doesn't bother me, as it seems to bother others, particularly in reference to the Narnia books. I love those books with a deep and abiding passion, and really never even noticed the Christian iconography until I was much older.
The reason why it doesn't bother me, I think, in both the Narnia books and this one, is that's it's not a preachy evangelical Christianity. C.S. Lewis' beliefs were not rigid or dogmatic - to him, (from his fiction, not having read his nonfiction) God truly is love. And that is all that is important.
So, Aslan in Narnia, Maleldil in this trilogy, the fact that Jesus is immanent as a presence, not really an issue for me.
So how does it stack up as science fiction? It's old-fashioned, for sure, reflecting nicely the age it was written in. There were certain echoes I caught of a short story I read by H.G. Wells - I can't remember the title. But the adventure, and then the discovery of a new planet, and particularly the idea that Earth is the benighted backwards planet in many ways, were all very enjoyable.
And his descriptions of the fullness of space (as opposed to the emptiness) were something I'd never seen before, and I always enjoy it when science fiction turns whole ideas on their heads.