I loved this book too and read it in practically one setting. I thought it worked really well both as sci-fi and thriller and had never seen the film so didn't know what was going to happen (although I could guess!)
I didn't think it felt dated at all, but I do read quite a lot of 'old' books so that's probably why.
I thought the characters were all solid and believable, but again, found it a bit lacking in the mothers' points of view. They all seemed to accept rather too easily that they weren't really their children and be happy to let them go which didn't seem entirely realistic to me.
This works as a great, fun read and also, as JackBauer says, leads onto more interesting questions about how this could be the next development of life on earth. It would be interesting to read alongside I Am Legend which deals with a similar sort of theme, although with a less hopeful ending.