The basic premise started out pretty simple: a Cinderella retelling... but she's a cyborg. It grew from there. At a most basic level, I find fairy tale retelling immensely enjoyable, and this was no exception. I read it start to finish in about 3 hours. (I'm a pretty fast reader... and I had a wide open evening for the first time in quite a while. Can you really blame me?) That ought to be a good sign, since it means I never got bored with it.
Overall, I found the story quite engaging. The world is well fleshed out, and Cinder is a sympathetic and (mostly) believable character. I really, really liked the premise and the fact that the story was more than just a retelling; it wove in a new plot line or two of its own. The rich setting, the addition of the Lunars and the plague to the world's history, and the introduction of cybernetic technology all add up to a pretty awesome backdrop for the story, and they mesh really well.
Was it perfect? No. I called a plot twist about 200 pages before the character found out, and I was certain she was going to find out right after I figured it out. But at the same time, those next 200 pages flew by, which made the (in my mind) unresolved reveal much less annoying. (In fact, I didn't even notice it had been so long until after Cinder actually found out.) There were a few bits here and there where the prose wasn't as polished as well and things like that (mostly little stuff that I probably only notice because I've been in my husband's writing group for a few years now). But the book as a whole showed a ton of potential for Marissa Meyer as an author, and I expect even greater things with every new book she publishes.
This is, by the way, the first in a series. There's not much in the way of resolution with a big tidy bow tying everything up at the end. I was left with enough of a conclusion that I was still content, but there are many things left unresolved that promise some pretty awesome potential in future books. Meyer strikes a nice balance of cliffhanger and closure that I found particularly satisfactory.