But there it is: "comics’ first 6,000 page graphic novel".
Yes, Sim's an arrogant, ill-tempered, misogynist prick, but he's still managed quite an accomplishment. Cerebus has had long stretches of tedium and moments of brilliance -- as normanrafferty put it, "Cerebus is both the paragon and nadir of self-publishing."
He makes some interesting points in the Farewell Letter linked above, one being that Independent Comics would never have happened without the Direct Sales market. That's an interesting counterpoint to the equally-valid observation that the Direct Sales Market contributes to the inbreeding in the industry, as comic publishers increasingly cater to the whims of an ever-dwindling coterie of hard-core fans while making little effort to cultivate new readers. The same forces that made independents possible at all also helped to turn the typical comic store into a monoculture of metastatized superhero titles, impenetrable to the casual reader.
In any event... someday, I may have to sit down and read the whole magnum opus, even the tedious bits. Since Sim has collected every single one of those 300 issues into thick "phone book" collections, that actually won't be as difficult or expensive as reading the complete runs of many more recent and shorter comic story arcs.