TL;DR: If you're looking for a quality Mexican dining experience or an excellent margarita, I highly recommend Border Grill.
A good restaurant is judged primarily on quality of food and quality of service (lack of either will ruin your dining experience), and the Border Grill at Mandalay Bay has both in spades.
I ate here with a party of 6, and we ordered several different dinner entrees as well as some appetizers - get the guacamole and chips, it's incredible - and drinks, and the entire party was very impressed with how attentive the wait staff was. Our glasses were never empty, but we also weren't bothered by anyone hovering asking us six times if everything was okay.
Our main server made excellent recommendations, and was very personable. Everything we ordered was fantastic. Special shout out to the manager, Daniel, who impressed us by checking on the tables - you know a place is going to have great service when the manager is around and visible.
I absolutely recommend this place if you're looking for Mexican food or somewhere cozy and relaxed to grab a drink!