Des Esseintes' philosophy can be summarized thusly: The era of Nature is over, and the artifice of a learned man can improve upon it in every way. To this end, Des Esseintes spends his time in strange pursuits, creating a hothouse of fake flowers and then, bored with that, a hothouse of real flowers that look fake. At one point, having become obsessed with the work of Dickens, he plans a trip to London, but having stopped on the way at an English restaurant in Paris, realizes that the artificial sensation of being in England has brought him as much satisfaction as he desires, and heads immediately back home.
Everything I've read about Against Nature suggests that Des Esseintes is a thinly veiled version of Huysmans himself. Much of this, I imagine, comes from the fact that the book is replete with dry, lengthy discussions of literature, art, and music which seem like the author's vain rambling. And to be sure, many of Huysmans' successors took Against Nature at its word, including Wilde, who refers obliquely to the book in A Picture of Dorian Gray. It's easy to see why Des Esseintes would become a guiding light for the aesthetes, since he spends so much of his time trying to make sure the colors of his walls are appropriate and matching (seriously, there are pages and pages of this), and yet it's hard for me to believe that Huysmans sees in Des Esseintes a paragon of humanity.
First and foremost, Des Esseintes' philosophy is frequently cruel. In the book's most famous episode, he has a jeweler inlay the carapace of a tortoise with gold and jewels, so that he might have a sort of mobile objet d'art. One night, he wakes up from a nightmare to find the tortoise dead:
He got to his feet to break the horrid fascination of his nightmare vision, and coming back to present-day preoccupations he felt suddenly uneasy about the tortoise.
It was lying still absolutely motionless. He touched it; it was dead. Accustomed no doubt to a sedentary life, a modest existence spent in the shelter of its humble carapace, it had not been able to bear the dazzling luxury imposed upon it , the glittering cape in which it had been clad, the precious stones which had been used to decorate its shell like a jewelled ciborium.
In another episode, Des Esseintes recalls a time in which he (thankfully unsuccessfully) devised a plan to turn a young boy into a murderer by bankrolling his expenses at a brothel until he became addicted, and then cutting off his patronage, forcing the boy to turn to drastic measures. It is difficult to read this in a way that does not suggest Des Esseintes' philosophy taken to its most extreme ends, triumphing over Nature but replacing it with a particularly grisly sort of Artifice. It's no wonder that certain rooms in Des Esseintes' home are decorated with Jan Luyken's engravings of religious suffering, or that he expounds his fascination with the sadistic element of Catholicism. It's possible that the the Aesthetes and Decadents may not have been repulsed by this, but it seems impossible that Des Esseintes, having appeared in 1884, would have gathered the same kind of following in Europe after World War I.
In any case, I think many of Against Nature's readers have overstated the parallels between Huysmans and Des Esseintes. Perhaps Huysmans sympathized with Des Esseintes' desire to escape a mediocre world, but his haven is ultimately unsustainable. Having begun to have olfactory hallucinations--imagining smells that are not there--and becoming weaker and weaker, Des Esseintes' doctors force him to return to the company of men. In the end, his disconnect from Nature nearly kills Des Esseintes, and he miserably ponders the only cure:
'In two days' time I shall be in Paris,' he told himself. 'Well, it is all over now. Like a tide-race, the waves of human mediocrity are rising to the heavens and will engulf this refuge, for I am opening the flood-gates myself, against my will. Ah! but my courage fails me, and my heart is sick within me!'