Generally when an author goes back to a character they wrote 10 years ago, especially to write a prequel, it won't be anything but unadulterated ****.
I haven't even thought of reading Ender's Shadow. Ender's Game was a good book, I don't want to spoil it.
One of Xenocide and Speaker for the Dead were OK, not great. I don't remember which. Neither spoiled Ender's Game though.