For me, XS gets the rare 5 star club rating. It never gets too crowded (even with big-name DJs) and I love how you can walk around and enjoy the music outside as well as inside. The outside area is spacious. Bottle service tables are inside and outside. Near the pool there is a bar and casino table game area. There are also 2 bars inside; one on each side of the stage. The dance floor has air conditioning blasting on it so you're not likely to leave the club covered in sweat. Since the club opens up to the pool area it also never gets stuffy. The sound system is great. The lights and production value aren't on par with other Vegas clubs but the other positives make up for it.
Guys might as well buy their tickets ahead of time. This time when I went, the pre-sale ticket line was near the entrance to Encore. You end up entering XS through the back. Other times when I have gone, the girls-only guest list line was in that spot. You do have the option to do a girls and guys guest list with a good ratio, but the line was already long at 10:30. My friend and I were on the girls only guest list and got inside in less than 1 minute. Last time it only took about 5 minutes of waiting in line. The bouncers here aren't rude (as far as I have experienced) and even wished me a happy birthday upon checking my ID.