It's an interesting essay, certainly more interesting than Lipkin's vitriolic review (which is far more vitriolic regarding Mieville's fans than towards the man himself). While I can certainly understand the frustration both writers express with CM's novels, I find that the very density of his books, and what may seem like opaqueness, is a part of their strengh. While every writer needs a good editor, I don't think Mieville needs to be reigned in.
I don't feel CM is the savious of Fantasy literature, as I believe it was well on the way to salvation before Perdido Street Station was published. I do believe he is the single most important voice in Fantasy right now, and a writer whose originality, imagination and plotting are, occasionally staggering. Mieville is a "big thinker", in a mode seen far more often in SF than in Fantasy. I've been saying for several years now that the scales of quality and originality in speculative fiction have been tipping towards Fantasy. I believe PSS tipped those scales, bringing a certain finality to a shift created by books by Martin, Kay, Swanwick, Gentle, Powers, and Gaiman among others.
CM's books are flawed, no doubt. Getting through the prologue of Perdido Street Station is hard work, and the first 150 pages or so of The Scar are certainly slow going. But by the time one is finished reading these books, the flaws are, if not erased, certainly diminished by the grandeur and scope of the story one has just read.
I've given lectures in conventions here in Israel for the past few years, about genre history, trends, sub-genres and so forth. And Perdido Street Station has come up repeatedly as an important road marker in genre literature. This is why readers forgive Mieville's flaws (while hoping he grows out of them. Iron Council, one should recall, is only his fourth novel). His books are simply too good, too overwhelming, to let these flaws stand in the way of our appreciation of them.