Ender's Game seemed like a "stand alone" to me. Similar to the Matrix, it was so well done that I could enjoy it over and over again. .
I read Ender's Game first and then read Ender's Shadow. Ender's Game gets my vote for being one of the best books i've ever read but Ender's Shadow seems forced, over analyzes everything and I have the impression the author is talking to me, explaining things instead of letting the plot take it's course.
I'm afraid Ender's Game was OSC best book and he hasn't been able to reach that same lever of storytelling again.