The books are fun to read. That's the most important thing. The characters are mostly likeable, the plot twists and turns keep me guessing, problems are resolved believably.
And as a writer, I am insanely jealous of the complex universe she's created with its infinite possibilities.
Begin with a collection of genetically engineered children, some of whom can influence computers with their minds, and a corporation that's using them to explore gates to alternate worlds; in this telling, there are infinitely many worlds more or less like our own, but in a tree where splits occur with major astronomical events.
I don't want to introduce too many spoilers, but I have to tell just a little more to illustrate just how fine this device is:
Exile all genetically engineered beings to one of these alternate worlds, an unpopulated one, along with the non-engineered families who want to go with their children. Let them blow up the gate on their way out so that they're truly alone. Show them building a new society.
Jump forward a millenium or so, and look at the countries that have been formed and how descendants of the genetically engineered do or don't fit into these societies. By now, they've developed a lot of interesting abilities, and they call the collection of skills "magic" (you know, as in "any sufficiently advanced technology is indistinguishable from magic." Thirteen of the original exiles appear to be functionally immortal, but due to brain damage during the exile they don't have very clear memories of how they got here. The collective subconscious has fitted them into archetypes and declared them to be gods. So as well as witches, wizards, and mages, you have the God of War, the God of Peace, the Goddess of Health and Fertility, and so on.
And you get some extremely amusing consequences when the God of War and the Goddess of Health get drunk together and decide to create a self-replicating wine containing all the useful spells they can think of.
Before the possibilities of Comet Fall, this new world. are even partially explored, Uphoff throws a wild card into the mix: Earth regains contact with the world of Comet Fall and decides to treat them like all the other worlds they've made Gates to, as a resource to be abused and exploited. (Hint: this does not work out well. For Earth.)
Getting dizzy yet? Here's another wild card: there is a third world with a small population of genetically enhanced people and a religion built around them. And they're interested in Comet Fall, too.
By Book Five everybody is spying on everybody else.
It's a beautiful setup. Uphoff starts small and then keeps expanding the original worldview. The possibilities for interaction between "magic" and engineering, the conflicts between the countries of Comet Fall, the conflicts between worlds provide an inexhaustible source of stories. And oh yes, there's another asteroid on a collision course with Comet Fall...
You see why I envy this? It's every sf/f writer's dream to build a world with so many possibilities that you never run out of stories to tell.