Rear mez is quite a large area so it really depends on where you are.
Because i go see phantom all the time i am happy with back row, literally, but it is very very far away so don't expect to be able to make out faces and all that, however your view is unobstructed unless you get some giant sitting infront of you.
for the 25th anniversary I sat in the front of the rear mez (right behind the asile) and those seats were fine and you didnt feel to high.
Basically the back of the rear mez is like at the same level as the chandelier
http://www.flickr.com/photos/27199361@N08/ Phantom at the Royal Empire Theatre