Personally, I think they're very good. The first, "Day of the Vipers", is on my top-five list, and the remaining two are also impressive. As for "knowing what happens", that's not the same as knowing
how or
why it happens
"Day of the Vipers" expands on the Cardassians wonderfully, and also Dukat individually, and features one of the best portrayals of the Bajorans I've yet seen.
"Night of the Wolves" and "Dawn of the Eagles" also handle the Bajorans well, and give particular insight into female Cardassians to balance the more frequent exploration of male Cardassians (there are distinctions in social role that were definitely worth exploring).
Basically, the "Terok Nor" books add even more complexity to the Bajor-Cardassia story. They're everything a prequal should be, in my opinion, particularly "Day of the Vipers". They flesh out what we already know well, throw out several good surprises, clarify details that were hinted at by the series in very interesting ways, and make us care about new characters as well as portraying the early lives of several existing ones.
Plus, the Oralian Way is in it. Yay!