My players went to the Plane of Mirrors quite some time ago. I don't remember the exact mechanics now (I'm at work), but I remember for some reason each wound up fighting his/her reflection alone. Most of them got stomped, but it was really boring and I was just rolling better. It would have been really fun, though if I'd killed one of them and inserted the evil version of the character into the group w/o them being any wiser about it. Depending on which player it was, I'd probably just take their character sheet and write the opposite alignment on it and say, "only you and I know." *wink* "let's see how long we can keep it that way."
On the whole, though, if I were going to do it again, I'd re-work the rules for that plane so it created your ideal opposite, not your alignment opposite. The hale & hearty fighter's mirror image is a sickly but cunning evil mage, etc.
Z