I haven't re-read it this time around but I did read it a few years back and enjoyed it with some reservations.
It is a little frustrating that some of the "why?" questions aren't better dealt with, as they would be in "good" SciFi but Ishiguro isn't really interested in the SciFi aspects of the book, it's just a device, a metaphor to allow him to talk about mortality. And he does that rather well, with well-drawn characters and affecting relationships.
Here's Ishiguro talking about the novel when the movie came out.