I read the entire trilogy. This is not the first of Gregory Keyes novels/stories I've read. I feel rather strongly that Greg always starts amazingly - The waterborn had one of the most electric beginnings I've experienced in recent times - I rate the possibilities of Perkar high up with Rand Al-thor and his mates in the Rob Jordan series.
For some reason - the waterborn part II fell short. I felt very strongly that the concluding parts of the kingdom series also left a bit to be desired - It was rammed with possibilities that (to me) seem'd to fade towards the end. I also felt that his need to "realise" (meaning make real) a fantasy story messed up the story a bit for me - I wasn't particularly keen on the people having come through a portal, and the entire sci-fi allusion. It's fantasy, there's absolutely no need to explain everything, just enough to have a good story.
In the end, I rate Greg highly I read another one of his stories - Age of confusion? Newton's Cannon, etc. That was truly fantastic and ground-breaking.
I think Gregory Keyes is an amazing story teller. I however feel that he is somewhat limited in the qualities of the stories he has to tell, which is a bit disappointing. His stories would be excellent for many writers, but for his writing skill, I guess I kind of expect just a little bit more.
I rate him just somewhat below GRRM and RJ - waaay above average