I have just finished DT for the first time, enjoyed it and would have been very glad to have more of it! I am no literary critic, but it has very much the Lewis feel to me. As someone mentioned above, the darkness, or the sexual tone in some parts, is nothing that is not found in some of his other stories. Ransom seems pretty much the same person as in OOTSP, before he is sent to Perelandra and is transformed there.
On critiquing literature:
"Do you mean to say you like it?" [Scudamour] cried.
"Like it?" said Knellie thoughtfully. "Does one like great art? One responds -- percieves -- intuits -- sympathises."
This is why I did not do well in English class, though I loved reading -- the ultimate to me was whether I liked or disliked what I read, preferring not to analyse and write papers blathering about this and that in the book.
Though I wonder what may correspond in our time/space to the many-bodied idol that Lewis says is still in the room?
Needess to say, I like Jack's books very much and hope he is still writing while in heaven. Perhaps he has figured out how to perfect and finish this story!