Not Ballenced, Not good.
This is a game of little, if any strategy at all. What you have made is a game of chance. I don't understand how this game is getting as good of rating as it's getting as anyone who really plays chess would abhor this. You could have every pawn on the board as your own but one, yet if they had a Medusa they would win.
My suggestion to you is remove the chance of the peices being yours or the other.
Make an array of pieces that you can roll from, each side equal. ( So you have a random chance of what peice you will get but you can only get so many of each.. equal to the other player.
Also figure another way to handle who has more power then the other.. Perhaps add hit-points and remove the "This guy can't attack this guy".
This would balance out the game considerably and give you a more strategic approach then winning.
I give it a four because I like the flash itself, I just don't like the game at all.