This one is an oddity for me. After a period in the 1990's wherein I was very cognizant of the whole sci fi scene and had read Cherryh extensively, the early 2000's presented issues for me and I drifted somewhat. Thus, when this was published I was absolutely unaware of it and it took a nudge from a good friend to alert me to it. Unfortunately, all I had available was a poorly scanned OCR copy of the book which definitively turned me off it and meant the book left no lasting impression.
Thus, this is an almost first read and it comes in at a very strong 4.5.
This is a much tighter tale than Cyteen, in scope and in timeline. Thus, the Michener comparisons fall away. The focus here is on transfer of power and the old adage that 'rebellions eat their young'. Compounded by one of the pivotal technologies of this setting being a drug that can extend lifespans to about 150.
Whilst the peril to the protagonists (same ones as in Cyteen) is ramped up considerably here, it ends up a far more optimistic book in some respects. A major theme is the peril to these societies. Earth has billions. Alliance and Union only total millions, between them. How to expand to avoid the 'eggs in one basket' issue without introducing new perils is a nice bit of speculation in here.
I went through this fast and hard, it was very good if not quite 5 start. I can't put my finger on exactly why and it may just be that it doesn't stand alone. You have to have read Cyteen to get anything from this book.
Still, a strong 4.5 is a strong recommend from me.