XS is by far one of the hottest clubs in Vegas right now. I went here on a Friday night, and the experience was absolutely amazing.
I arrived there with my girlfriend around 11:15am. We hadn't purchased tickets ahead of time and consequently had to wait in the normal line for about 1 - 1.5 hours. Cover was $50 for guys, and $20 for girls, but it was so worth it. I would definitely recommend buying tickets ahead of time if you can; you get to skip the huge normal line and go right to the front with a ~5 - 10 minute wait.
The club was super packed inside and consequently insane. I bought 2 AMF's, which were $17 each and pretty strong (a good thing if you're paying $17 for a drink...). Hard Rock Sofa was playing that night and everyone was super into the music, fist pumping, and having a good time. If you're in Vegas and in need of some good clubbing action with some loud EDM, be sure to check out XS. It's a must.
Tip: buy tickets online to avoid waiting 1 - 1.5 hours!