I have the occasional argument with friends about whether Tad Williams has anything of his own to say. While I agree that Memory, Sorrow, and Thorn was derivative, I thought Otherland was innovative and interesting, as was The War of the Flowers. The first two books of Shadowmarch seemed to bear out that promise.
Shadowrise, unfortunately, bogs down substantially. It's not bad, but it is dull. The complex plot moves on, and Williams does a good job of keeping all the key characters in play. Potentially intriguing things happen, but somehow they're not really intriguing. For one thing, he doesn't really explore them - they just happen, then are left behind. While all the characters are on desperate, life and death quests, the book somehow becomes just a series of events - "this happened, then this happened, then that happened, then the other thing happened". Even the complex relationships of the gods, which we learn more about, didn't really hold my interest. One revelation apparently intended as a key tipping point in the human-fairy relationship evoked no more from me than acknowledgement - because I never got very deeply involved.
The book isn't bad - it just feels like filler. Williams notes that he intended it to be shorter, and it probably would have been a better book that way. One case when one wishes for a firmer editorial hand.
I already have Shadowheart, book 4 of the series, but I feel I can fairly well predict what will happen in that heavy volume. I foresee more of the same - moving, complex parts, but no great surprises. I hope I'm wrong.
If you're considering whether to read past book 2, and you're not a die-hard Williams fan, I'm afraid I have to advise against. Too big an investment of time for too little return.