I agree with Cintya P.'s review below and I was actually there the same night. I gave it one extra star because our free passes that I purchased off ebay worked to get us in for free (for girls only), and I just had an all around great time!
The one bad thing about XS is that it gets EXTREMELY crowded on the dance floor, so it's really hard to get in and dance with your group without getting hit from all sides by other people dancing. BUT, we got there early and were able to dance for a good few hours before it got too crowded.
One tip: guys should buy their tickets ahead of time online. Our guy did this, with the group of 5 girls, and they let him right in with us. AND, get there early, like around 10pm.
Overall, XS and Marquee are the top 2 clubs in Vegas right now and they are entirely worth going to! BTW, XS is open Sunday nights, one of the few clubs around open on Sundays.