I liked Ender's Game, but I found Speaker for the Dead, it's sequel, to be a much more intelligent book. I had already figured out the ending of Ender's Game (the solution that had to be chosen) much earlier in the book, so it ended with a *hmmf* for me. Speaker for the Dead had a more fleshed out cast and a mystery that baffled me. I also liked the philosophy used in it. After Speaker, it was still well written... but felt like it was just continuing to be continued.
I'd read the first two, and judge from there.