I know what all (or most) of you are getting at, there's been a lot of times i've been halfway through one of his novels and it's seemed more of a chore than a pleasure - you get lost in his dense, labyrinthine sentences, constantly have to read back on yourself, you lose track of the ridiculous number of characters - not to mention the amount of maths and science involved which often goes totally over my head (the guy's actually a rocket scientist, for chrissakes) etc etc. There probably hasn't been one novel of his where i haven't experienced the feeling that it's an exhausting slog at least once. And i know (well, from what i've read) that most dissensers are pretty much over with post-modernism, so i can understand why they'd find the mass jumble of stuff going on a bit tiresome.
But... there's just so many things I love, too many to mention, and unfortunately i suspect that all the reasons i love his stuff are going to be exactly the same reasons why you guys don't...
I love his prose, which i think sometimes is just as beautiful as Nabokov's - for instance the passage in Crying of Lot 49 where he's describing LA from above at night, comparing it to a computer circuit board (can't remember the exact quote sadly)... i agree with Idlerich that it can be pretty fiddly at times, but that's exactly one of things i love about it. I love the characterisation and dialogue, the silly humour and puns, the songs, the kazoos, the vast range of styles he moves through (i know, i know, pomo), the ridiculously detailed research that goes into it and all the obscure references (there was a bit about "the real Inspector Sands" in Against the Day recently, and a Tetris moment earlier in the novel), the historical settings, the amazing synthesis of science/science fiction, fantasy/reality... i know i'm just listing stuff here and not actually providing any meat to back myself up, but i could really just prattle on all day if i'm not careful. I even love his essays and articles, which i definitely recommend reading if you haven't done so -- the introduction he wrote in a recent edition of 1984 was really good.
I think my favourite of his stuff is Mason & Dixon, and that's kinda unique in relation to the rest of his stuff, in that it follows just two central characters - and i think they're definitely his most 'fleshed-out' characters. An amazing book... and short enough that you can finish it (fairly) easily!