I find At the Mountains of Madness to be one of the sloggy ones.I'm the same... I always sum him up as 'great ideas, kind of a turgid writer, despicable politics'. That said, there are a couple of his stories that I found to be fascinating, particularly "At the Mountains of Madness"...
When the PCs in my Traveller game were excavating an alien installation frozen in ice, which they were suspecting might have strange psionic tendencies and lurking dangers, I suggested to one of the players who reads a lot of sci-fi that he should read At the Mountains of Madness to get into the zone. I don't know if he's forgiven me yet for that particular bit of advice . . .
There are things that humankind was not meant to experience . . . like long accounts of Antarctic latitudes and longitudes, and page-long descriptions of Pabodie's amazing drill.