I liked all of the Chronicles, but this was the one where she really let loose with the sci-fi. I think I've read it three times and I still can't exactly picture what a Ket is supposed to look like, or how their society really works.
That said, yeah, the ending is sort of sad, but wasn't there a whole "But perhaps I am just a puppet in some even higher being's game, and who knows what happens next?" part to it too? So yeah, Rachel left *our* space time continuum, but that doesn't mean it's hopeless.
I guess.