Some Cherryh fans would probably file intent if they saw that I rated this series 4-1/2 stars, instead of five. But I could not quite justify five stars. There are Cherryh books that are better than Foreigner. Perhaps because of its extraordinary length, the quality is sometimes uneven. Defender was particularly weak. Destroyer has similar flaws; though there's a bit more action, it's so very similar to the action we saw in the first trilogy.
The series took awhile to grow on me. I found the first book mediocre; entertaining, but nothing to write home about. It was just barely entertaining enough to get me to buy and read the second one. But after the third one, I was hooked. So far, the third and fourth books, Inheritor and Precursor, are my favorites.
C.J. Cherryh's Worlds - C.J. Cherryh's official site.
Shejidan - The name of this site is from the Foreigner series, but it's a general Cherryh site, with art, discussion, etc. IMO, the best Cherryh site on the net.
A Cherryh Bibliography - Fantastic Fiction's complete list of C.J. Cherryh's books.