I've read the majority of his stuff (minus Tailchaser's Song and the last two Bobby Dollar novels, and a few other things.) I'm a big fan.
I read Otherland at the right time (not long after it was released), so it was prescient, cutting edge, and has really stuck with me. A wonderful set of core characters, that still remain clear and unique in my head a decade later. I'm not sure how well the technological and social aspects hold up these days--though, I wouldn't be surprised if they're just as relevant now as then.
I actually came to Williams through the Shadowmarch series after bouncing off Memory, Sorrow, and Thorn 3-4 times. It's a lot more approachable, and Williams' prose style is more consistent than MST (which, frankly, is all over the place, especially during The Dragonbone Chair.) It really comes into its own in the third and fourth books, and would scratch the itch for anyone looking to read an epic, well developed fantasy in a fascinating world. It doesn't quite hit the modern classic level of MST, but it's certainly worth your time.
Yeah. I reread The Dragonbone Chair late last year, but I don't know if I'm going to be able to squeeze in the last two novels. They're so bloody long, and not exactly quick reads.