Well, it was both his final tribute to and repudiation of the Dunsanian mode. It was also, in many ways, his "spiritual autobiography", both symbolically and literally (appearances of such things as the night-gaunts and the ghouls, etc.; and their changing roles).
Personally, it has always been among my favorite Lovecraft pieces, since I first read it in At the Mountains of Madness and Other Novels, from Arkham House. I would agree that it should be taken (at least first time around) in smaller doses, rather than all at one sitting, as this particular type of tale can overwhelm by the nature of the voice, or with the procession of "wonder" after "wonder", so that each begins to lose its effect (for some, at any rate) -- a problem that Lovecraft himself was concerned with, which is one of the reasons he never attempted to even type it up, let alone submit it for publication.
There are certain problems with the book (for me), such as some of the names for various types of beings encountered, which have something of a juvenile flavor. One can (quite plausibly) argue that this is fitting with the absurdist nature of a dream narrative at times, or with the fact that some of these do come from Lovecraft's own early childhood dreams and the like; but for me, the tone of these names still forms a flaw (albeit a minor one, sort of like Eddison's "Demons", "Witches", "Pixies", and so forth, for his different peoples in The Worm Ouroboros).
Nonetheless, I've always found this one to be a superb work of the imagination, one which -- as with so much of Lovecraft -- grows ever larger with each reading, and one which in many ways is a very moving tale (especially the ending).
(On the other hand James Blish, among others, gave the novel very short shrift indeed....)
I'll be interested in hearing your thoughts once you're done....