The Otherland quartet holds up very well throughout all four books. It doesn't stumble horrifically like some series *cough* WheelOfTime *cough*, though it lacks some of the depth that I hoped for when I initially started reading. There was a lot of potential for some very interesting philosophical speculations on the technology he dealt with, but in my opinion he never quite gets there. Sellars' secret, in particular, seems to be more done as an off-hand swipe at the issue without really getting into the meat. I suspect this is because TadWilliams? lacks a really strong technical background; he convincingly fakes it in many places, but occasionally even I noticed slight discrepancies and misunderstandings of the way computer systems work.
TadWilliams?' plotting in OtherlandSeaOfSilverLight, though tight, doesn't have quite the same impact as ToGreenAngelTower?: there is no sudden surprise twist at the very last moment. However, in many ways I appreciated the more graceful denouement. OtherlandSeaOfSilverLight is sort of curious in that it is very difficult to point to any small section of pages and call that the climax; the climax is curiously diffuse and seems to go on for a good latter third or so of the book.
My suspension of disbelief was tweaked a bit by one part of the book in particular, the revelation of the identity of the Other. I had been hoping for something more . . . alien, and the explanation he gave for the Other's powers seemed a bit too much like a deus ex machina for my comfort. I guess, though, that I perhaps should have been prepared for it.
All in all, the series is a fun piece of very light sci-fi, which isn't too terribly deep.