Just for the record, we have been to Wicked, X, M4, OZone and Happy Hedonist many times (we sometimes learn slowly). And to Abstract, Hers, Jade, 2250, Eros and others further in the past.
As far as the back room at X, we have never seen it busy. X was always more of an off-premises, poser (though quite attractive) crowd. We tend to go to the back earlier than some people do, we might go back at midnight. Typically we would be the only ones there, or perhaps there might be one or two other couples. It is a nice setup, though a little chilly for us. It is staffed by three young people, who stay at the front, laugh their asses off, and are oblivious to anything going on in the back. The change room can fit one couple, perhaps three people at most, so if it were busier in the back, the change room would be a bottle neck. The actual setup is pretty good, a long corridor with private rooms and some theme rooms, and a sophisticated open area at the back.
For us, Wicked is the standard for on-premises areas, lots of beautiful naked people to watch, it is big and nicely set up, and there is a bar. The big drawback for us is that they do not open it until 12:30 or so, so we do not get home until 3:30 or 4, and we are too old for that.