"You can make the reservation, but you cannot hold the reservation."
Here with a bachelor party last Saturday night and we had a table reservation for the crew. We got there at 10:30pm and they told us we we were too late (claimed we had to be there by 10pm) and they had given our table away. After some bickering they let us in and we found that half the tables in the club were unoccupied and they were most definitely not sold out.
With some pushing we eventually claimed a table and the night was saved. My poor friend who organized this part of the bachelor party weekend definitely lost a few hairs from the stress of the table initially falling through.
XS charges way too much money to screw up like that, so even though it's one of my favorite party spots in Vegas they only get 3 stars.